Horowitz Seminar on Probability, Ergodic Theory and Dynamical Systems


The seminar covers a wide range of topics in Pure and Applied Probability, Mathematical Physics, Ergodic Theory and Dynamical Systems. The seminar is organized by Ron Peled.
Usual place and time are Tel Aviv University's Schreiber Building room 309, on Mondays at 14:30-15:30, but check each announcement since this is sometimes changed.
To join the mailing list write to peledron (* the at symbol *) post.tau.ac.il.

The seminar is named after Shlomo Horowitz, who was among the first faculty members to study probability theory at Tel Aviv University.




Spring Semester 2017


Monday, March 13

Mike Hochman, Hebrew University of Jerusalem
How to efficiently code irregular sequences
Abstract:
Claude Shannon showed that entropy gives the optimal per-symbol coding rate for typical samplkes taken from stationary stochastic processes. Such a sample displays statistical regularity, e.g., the empirical frequencies exist, and describe a sigma-additive probability measure. What happens for sequences without such regularity? In the talk I will answer this in the setting of Borel dynamics and explain why every Borel automorphism without invariant probability measures can be coded with arbitrary efficiency. I would like to focus on some specific examples and explain some of the methods goind into coding of sample paths whose empirical frequencies don't exist.
Schreiber Building Room 309 at 14:30.

Monday, March 20

Sébastien Martineau, Weizmann Institute
Locality of critical parameters in statistical mechanics
Abstract:
Cayley graphs allow us to get graphs from groups, generalizing the process of getting the hypercubic lattice from Zd: take a group G generated by a finite set S, take the vertex set to be G and put an edge between any g and gs, where g is in G and s is in S. Now, consider a way to assign to each Cayley graph a real number. Can this number be well computed if you are only given the ball of radius 1000 of the Cayley graph under study? This is (essentially) the locality question. I will review the status of this question for the connective constant and the critical parameter of Bernoulli percolation, and explain an application of such results.
This talk will be as self-contained as possible. Includes joint work with Vincent Tassion.
Schreiber Building Room 309 at 14:30.

Monday, March 27

Leonid Pastur, Institute for Low Temperature Physics & Engineering, Ukraine
Analogs of Szegő’s Theorem for Ergodic Operators
Abstract:
We present a setting generalizing that of Szegő’s theorem on the Töplitz operators. Viewing the strong Szegő’s theorem as an asymptotic trace formula, which is determined by an underlying Töplitz operator and by two functions (the symbol and the test function), we replace the Töplitz operator by an ergodic operator (e.g. random or quasiperiodic). In the framework of this setting we consider a variety of asymptotic formulas different from those known for the Töplitz operators and including certain central limit theorems and formulas related to quantum information theory.
Schreiber Building Room 309 at 14:30.

Monday, April 3

Evgeny Strahov, Hebrew University of Jerusalem
Discrete time determinantal processes related to products of random matrices
Abstract:
I will talk about a family of random processes in discrete time related to products of random matrices (product matrix processes). Such processes are formed by singular values of random matrix products, and the number of factors in the random matrix product plays the role of a discrete time. I will explain that in certain cases product matrix processes are discrete-time determinantal point processes, whose correlation kernels can be expressed in terms of double contour integrals. In particular, this enables to investigate different scaling limits of the determinantal process under consideration.
Schreiber Building Room 309 at 14:30.

Monday, April 10 and Monday, April 17

Passover break - No seminar.

Monday, April 24

Lei Yang, Hebrew University of Jerusalem
Badly approximable points on curves and unipotent orbits in homogeneous spaces
Abstract:
We will study n-dimensional badly approximable points on curves. Given an analytic non-degenerate curve in R^n, we will show that any countable intersection of the sets of weighted badly approximable points on the curve has full Hausdorff dimension. This strengthens a previous result of Beresnevich by removing the condition on weights. Compared with the work of Beresnevich, we study the problem through homogeneous dynamics. It turns out that the problem is closely related to the study of distribution of long pieces of unipotent orbits in homogeneous spaces.
Schreiber Building Room 309 at 14:30.

Monday, May 1

Memorial day - No seminar.

Monday, May 8

Dominic Yeo, Technion
Frozen percolation with k types
Abstract:
We study a model for the effect of infections on a population where individuals have one of k types. An inhomogeneous random graph represents the initial connections between these individuals, and over time new connections are made homogeneously, as in the classical random graph process. Each vertex is infected at some rate, resulting in the removal of its entire component. This is a version of a frozen percolation model which (under mild conditions) exhibits self-organised criticality: the dynamics first drive the system to a critical state, and from then on maintain it in criticality. We prove concentration results for the sizes of the components and a local limit theorem, in terms of a multitype branching process whose parameters are critical and described by the solution to an unusual differential equation.
Schreiber Building Room 309 at 14:30.

Monday, May 15

Hao Wu, Université de Genève
Hypergeometric SLE and Convergence of Critical Planar Ising Interfaces
Abstract:
Conformal invariance and critical phenomena in two-dimensional statistical physics have been active areas of research in the last few decades. This talk concerns conformally-invariant random curves that should describe scaling limits of interfaces in critical lattice models.
The scaling limit of the interface in critical planar lattice models with Doburshin boundary conditions (b.c.), if exists, should satisfy conformal invariance (CI) and domain Markov property (DMP). In 1999, O. Schramm introduced SLE process, and this is the only one-parameter family of random curves with CI and DMP. In 2010, D. Chelkak and S. Smirnov proved that the interface of critical Ising model on the square lattice does converge to SLE(3). In this talk, we discuss the scaling limit of the pair of interfaces in a rectangle with alternating b.c. The scaling limit of the pair of interfaces, if exists, should satisfy CI, DMP and symmetry (SYM). It turns out there is a two-parameter family of random curves satisfying CI, DMP, and SYM, and they are Hypergeometric SLE. For the critical Ising model on the square lattice, the pair of interfaces does converge to Hypergeometric SLE(3). In this talk, we will explain two different proofs for the convergence. Furthermore, we will discuss results about global and local multiple SLEs, which correspond to the scaling limit of the collection of interfaces with alternating b.c. in more general setting.
Schreiber Building Room 309 at 14:30.

Monday, May 22

Barak Weiss, Tel Aviv University
Random walks on homogeneous spaces and diophantine approximation on fractals
Abstract:
We extend results of Y. Benoist and J.-F. Quint concerning random walks on homogeneous spaces of simple Lie groups to the case where the measure defining the random walk generates a semigroup which is not necessarily Zariski dense, but satisfies some expansion properties for the adjoint action. Using these dynamical results, we study Diophantine properties of typical points on some self-similar fractals in R^d. As examples, we show that for any self-similar fractal K satisfying the open set condition (for instance any translate or dilate of Cantor’s middle thirds set or of a Koch snowflake), almost every point with respect to the natural measure on K is not badly approximable. Furthermore, almost every point on the fractal is of generic type, which means (in the one-dimensional case) that its continued fraction expansion contains all finite words with the frequencies predicted by the Gauss measure. Joint work with David Simmons.
Schreiber Building Room 309 at 14:30.

Monday, May 29

Eliran Subag, Weizmann Institute
The geometry of the Gibbs measure of pure spherical spin glasses
Abstract:
How does a random function on a manifold of very high dimension typically look like? The talk will focus on aspects of this question for the pure spherical spin glass models of statistical mechanics -- namely, random homogeneous polynomials restricted to the N-sphere. First, we will see how the second moment method can be applied to study the distribution of critical points at a given height. Then, we will describe the limiting distribution of the point process associated to critical values (a joint work with Ofer Zeitouni). Finally, we will describe a geometric picture for the Gibbs measure at low temperature: as the dimension tends to infinity, the measure concentrates on "bands" around the critical points of highest values.
Schreiber Building Room 309 at 14:30.

Note special day and time!
Sunday, June 4

Noam Berger, Hebrew University of Jerusalem
Harnack inequlity for balanced environments.
Abstract:
We consider random balanced, not necessarily elliptic, difference equations, and prove a Harnack inequality in the i.i.d. case. We discuss the relation of this result with random walk and percolation. We then discuss non-i.i.d. cases, and, time permitting, discuss the conjectured continuous analogue of this result.
Room TBA, at 12:10.

Monday, June 12

Matthew Kwan, ETH Zürich
TBA
Schreiber Building Room 309 at 14:30.

Monday, June 19

Van Vu, Yale University
TBA
Schreiber Building Room 309 at 14:30.

Monday, June 26

Omer Tamuz, Caltech
TBA
Schreiber Building Room 309 at 14:30.





Fall Semester 2016


Monday, October 31

Xiaolin Zeng, Tel Aviv University
Recurrence of two dimensional edge-reinforced random walk
Abstract:
The (linearly) edge-reinforced random walk is a self-interacting random walk in which, at each step, the walker prefers traversing previously visited edges, with a bias proportional to the number of times the edge was traversed. Since its introduction by Diaconis and Coppersmith (1986), the question of understanding the long-time behavior of the walk has received significant attention, and it is natural to expect that the walk is recurrent on recurrent graphs (when simple random walk is recurrent). We prove that the walk is recurrent on Z^2 (for any constant initial weights). All terms will be explained in the talk. Based on joint works with Sabot and Tarres.
Schreiber Building Room 309 at 14:30.

Monday, November 7

Romain Tessera, Université Paris-Sud
Limit shape theorem for first passage percolation on nilpotent groups
Abstract:
In a joint work with Itai Benjamini, we prove a limit shape theorem for first passage percolation on a nilpotent group. The idea of proof can be used in the classical setting as well, providing a new (essentially geometric) proof of Alexander's estimates on the speed of convergence to the limit shape.
Schreiber Building Room 309 at 14:30.

Monday, November 14

Paul Smith, Tel Aviv University
Towards universality in bootstrap percolation
Abstract:
The study of the long-term evolution of monotone cellular automata on lattices (subject to random initial conditions) dates back to the late 80s, although it is only recently that a general definition of such `bootstrap percolation' models has been given and an ensuing theory begun to be developed. The key question that one would like to answer is: to what extent do these models, which may look completely different at the microscopic scale, behave in `essentially' the same way at the macroscopic scale? In this talk I shall answer (a precise version of) this question in two dimensions, and, time permitting, mention some recent work in higher dimensions.
Schreiber Building Room 309 at 14:30.

Monday, November 21

Vitali Wachtel, Universität Augsburg
First-passage times for random walks with non-identically distributed increments
Abstract:
We consider random walks with independent but not necessarily identically distributed increments. Assuming that the increments satisfy the well-known Lindeberg condition, we investigate the asymptotic behaviour of first-passage times over moving boundaries. Furthermore, we prove that a properly rescaled random walk conditioned to stay above the boundary up to time n converges, as n tends to infinity, towards the Brownian meander.
Schreiber Building Room 309 at 14:30.

Monday, November 28

Vincent Delecroix, LaBRI, Bordeaux
Two dynamical generalizations of Hurwitz's theorem in Diophantine approximation
Abstract:
One way of stating Hurwitz's theorem in Diophantine approximation is that among rotations of the interval, the one that achieves the worst recurrence rate is the rotation by the golden mean. We propose two generalizations of this result in a dynamical context. First of all, Hurwitz's theorem holds in the class of all measurable transformations that preserve the Lebesgue measure. Secondly, if time permits, we will explain a generalization of Hurwitz's theorem in the class of interval exchange transformations. One important step in proving both theorems is a packing problem in 2 dimensions.
Schreiber Building Room 309 at 14:30.

Monday, December 5

Gordon Slade, University of British Columbia
Critical phenomena and renormalisation group
Abstract:
This talk is a continuation of the morning's colloquium, to enter into greater detail about the renormalisation group method.
Schreiber Building Room 309 at 14:30.

Monday, December 12

Yair Hartman, Northwestern University
Percolation, Invariant Random Subgroups and Furstenberg Entropy
Abstract:
In this talk I'll present a joint work with Ariel Yadin, in which we solve the Furstenberg Entropy Realization Problem for finitely supported random walks (finite range jumps) on free groups and lamplighter groups. This generalizes a previous result of Bowen. The proof consists of several reductions which have geometric and probabilistic flavors of independent interests.
All notions will be explained in the talk, no prior knowledge of Invariant Random Subgroups or Furstenberg Entropy is assumed.
Schreiber Building Room 309 at 14:30.

Monday, December 19

Percy Deift, Courant Institute, New York University
Universality in numerical computations with random data. Analytical results
Abstract:
This is joint work with Tom Trogdon. Here the speaker shows how to prove universality rigorously for the fluctuations in the stopping times for certain, standard numerical algorithms with random data. The proofs rely on recent state of the art results from random matrix theory.
Schreiber Building Room 309 at 14:30.

Monday, December 26

Igor Rivin, University of St. Andrews
Statistics of classes of random graphs
Abstract:
We will discuss the distribution of eigenvalues of various classes of random graphs,including geometric graphs, and Cayley and Schreier graphs of finite groups with respect to random generating sets.
Schreiber Building Room 309 at 14:30.

Monday, January 2

The Horowitz seminar is held jointly with the Action Now meeting at Tel Aviv University.
The meeting is from 9:30 to 17:00 and features Adrien Boyer, Lionel Levine, Yair Minsky, Yuval Peres and Boris Solomyak. See the schedule here.

Monday, January 9

Daniel El-Baz, Tel Aviv University
Angles in adelic quasicrystals and related problems
Abstract:
We discuss some results concerning the local statistics of directions in certain 'adelic quasicrystals'. One such local statistic is the gap distribution, where we build on work by Jens Marklof and Andreas Strömbergsson in the case of (affine) Euclidean lattices and quasicrystals, and by Noam Elkies and Curtis McMullen who studied the fractional parts of sqrt(n). Another is the pair correlation, where we build on joint work with Jens Marklof and Ilya Vinogradov. The methods involve homogeneous dynamics and analytic number theory.
Schreiber Building Room 309 at 14:30.

Monday, January 16

Omer Bobrowski, Technion
Topology of Random Geometric Complexes
Abstract:
A random geometric complex is an abstract simplicial complex whose vertices are generated by a random point process in a metric space, and higher-order simplexes are added according to some rules that depend on the geometric configuration of the vertices. In this talk we will review recent advances in the study of the homology of random geometric complexes. Loosely speaking, homology is a topological-algebraic structure that contains information about cycles of various dimensions in the complex. We will discuss phase transitions related to “homological connectivity”, as well as the behavior of these complexes in the thermodynamic limit and a higher dimensional notion of percolation.
Schreiber Building Room 309 at 14:30.

Monday, January 23

Roland Bauerschmidt, University of Cambridge
Eigenvalue statistics for random regular graphs
Abstract:
I will present results on local eigenvalue statistics for random d-regular graphs. The focus of this talk will be on results on eigenvalue spacing statistics when d belongs to [N^{o(1)}, N^{2/3-o(1)}]. The talk will be mostly complementary to my talk at the Workshop on Mathematical Physics on the next day, which will focus on results in the regime of fixed degree d, to which I will give a brief outlook (time permitting).
This is joint work with A. Knowles, J. Huang, and H.-T. Yau.
Schreiber Building Room 309 at 14:30.





Spring Semester 2016


Monday, February 1
Special semester break seminar!

Gaultier Lambert, KTH University, Stockholm
Mesoscopic linear statistics of determinantal processes
Abstract:
Determinantal point processes arise in the description of eigenvalues of unitary invariant Hermitian random matrices, as well as in many statistical mechanics models such as random tilings, non-intersecting paths, etc. I will explain a cumulant method developed by A. Soshnikov to analyze the asymptotics distributions of linear statistics of determinantal processes. In the mesoscopic regime, within the sine-kernel universality class, we prove that the fluctuations are universal and described by a Gaussian process with H^1/2 correlations. I will also discuss different models which exhibit a transition from Poisson to GUE where non-universal behaviors are observed.
This is a joint work with Kurt Johansson.
Schreiber Building Room 309 at 14:30.

Monday, February 29

Mikhail Sodin, Tel Aviv University
Spectral measures of {0, 1}-stationary sequences
Abstract:
We will discuss what is known and what isn't about spectral measures of {0, 1}-stationary sequences, and how these spectral measures are related to some objects of classical analysis. As usual, we know much less than we would like.
Schreiber Building Room 309 at 14:30.

Monday, March 7

Gidi Amir, Bar-Ilan University
Liouville groups with very slowly growing harmonic functions
Abstract:
A group G has the Liouville property with respect to some generating set S if the only bounded harmonic functions on the Cayley graph of (G,S) are the constant functions. On such Cayley graphs it is interesting to ask how slowly can a non-constant harmonic function grow? We construct groups with arbitrary slowly growing harmonic functions. More precisely, for any "nice" function f growing slower than log n, we construct a group and a generating set so that there is non-constant harmonic function growing like f, but any harmonic function asymptotically slower than f must be constant.
This is joint work with Gady Kozma.
Schreiber Building Room 309 at 14:30.

Monday, March 14

Aser Cortines, Technion
Finite-size corrections to the speed of a branching-selection process
Abstract:
We consider a stochastic model of N evolving particles described by a branching mechanism with selection of the fittest. The model can be seen as the infinite range limit of a directed polymer in random medium with N sites in the transverse direction. The particles remain grouped and move like a travelling front driven by a random noise with a deterministic speed. We focus on the case where the noise lies in the max-domain of attraction of the Weibull extreme value distribution and show that under mild conditions the correction to the speed has universal features depending on the tail probabilities.
Joint work with Francis Comets.
Schreiber Building Room 309 at 14:30.

Monday, March 21

Oren Louidor, Technion
Aging in a logarithmically correlated potential
Abstract:
We consider a continuous time random walk on the box of side length N in Z^2, whose transition rates are governed by the discrete Gaussian free field h on the box with zero boundary conditions, acting as potential: At inverse temperature \beta, when at site x the walk waits an exponential time with mean \exp(\beta h_x) and then jumps to one of its neighbors chosen uniformly at random. This process can be used to model a diffusive particle in a random potential with logarithmic correlations or alternatively as Glauber dynamics for a spin-glass system with logarithmically correlated energy levels. We show that at any sub-critical temperature and at pre-equilibrium time scales, the walk exhibits aging. More precisely, for any \theta > 0 and suitable sequence of times (t_N), the probability that the walk at time t_N(1+\theta) is within O(1) of where it was at time t_N tends to a non-trivial constant as N \to \infty, whose value can be expressed in terms of the distribution function of the generalized arcsine law. This puts this process in the same aging universality class as many other spin-glass models, e.g. the random energy model. Joint work with Aser Cortines-Peixoto and Adela Svejda.
Schreiber Building Room 309 at 14:30.

Double seminar! Note special time! Each talk will be 50 minutes.
Monday, March 28

Jon Aaronson, Tel Aviv University
Alexander Glazman, Université de Genève
Aaronson title: Distributional limits of positive, ergodic stationary processes & infinite ergodic transformations
Aaronson Abstract:
Every random variable on the positive reals occurs as the distributional limit of the partial sums some positive, ergodic stationary process normalized by a 1-regularly varying normalizing sequence (& the process can be chosen over any EPPT).
I'll try to explain this and (time permitting) some consequences for infinite ergodic theory. Joint work with Benjamin Weiss.
Glazman title: Properties of integrable self-avoiding walks
Glazman Abstract:
We consider a self-avoiding walk on the dual Z^2 lattice. This walk can traverse the same square twice but cannot cross the same edge more than once. The weight of each square visited by the walk depends on the way the walk passes through it and the weight of the whole walk is calculated as a product of these weights. We consider a family of critical weights parametrized by angle theta in [pi/3,2pi/3] which satisfy the Yang-Baxter equation. One should view a square with weights corresponding to theta as a rhombus with angle theta.
For theta=pi/3, this can be mapped to the self-avoiding walk on the hexagonal lattice. In this case the connective constant was recently proved to be equal to \sqrt{2+\sqrt{2}}. We show that this proof can be generalized to compute the asymptotic of the partition function for any theta in [pi/3,2pi/3]. Moreover, the 2-point function in the half-plane does not depend on the chosen rhombic tiling as long as all angles are between pi/3 and 2pi/3. This implies that the partition function of bridges of a fixed height tends to 0 as the height tends to infinity. The latter was recently proved for the self-avoiding walk on the hexagonal lattice. In this case we give a new simple proof relying on the parafermionic observable (partly joint work with I. Manolescu).
Schreiber Building Room 309 at 14:10.

Monday, April 4

Amos Nevo, Technion
Hyperbolic geometry and pointwise ergodic theorems
Abstract:
We will describe a completely elementary direct geometric proof of pointwise ergodic theorems for natural averages on the group SL(2,R). We will then describe how this result can be used to generalize the existing pointwise ergodic theorems for averages on isometry groups of hyperbolic spaces beyond the radial case.
Schreiber Building Room 309 at 14:30.

Monday, April 11

Erez Nesharim, Tel Aviv University
Existence of Badly Approximable Vectors in Fractals
Abstract:
In ergodic dynamical systems almost every point is generic. Many times it is interesting to understand how large is the set of non-generic points. In this talk I will present a criterion for a set to have a nonempty intersection with every “regular” fractal, and mention some applications in Diophantine approximation and dynamics on homogeneous spaces. This talk is based on a joint work with Badziahin, Harrap and Simmons.
Schreiber Building Room 309 at 14:30.

Monday, April 18 and Monday, April 25

Replacement day for Wednesday courses and Passover break - No seminar.

Monday, May 2

Tal Orenshtein, Université Lyon 1
Router walks on regular trees
Abstract:
Router Walk is a generalization of the Rotor Router model. Initially there is a router configuration on the graph, that is in every vertex there is an infinite sequence of routers which point to its neighbors. Given a router configuration, the walk is deterministic: at each step the walker follows the next unused router in its current location and jumps to a neighbor. In this talk, which is based on a recent joint work with Sebastian Mueller, we shall discuss the problem of recurrence and transience of router walks on regular trees with i.i.d. router sequences.
Schreiber Building Room 309 at 14:30.

Monday, May 9

Dmitry Ioffe, Technion
Finite range polymers with on-site repulsion
Abstract:
We discuss polymers on integer lattices, which are modeled by finite range random walks in repulsive potentials, and which are subject to pulling forces, confining geometries (e.g. half-spaces or cones ) and interactions with active substrates.
Based on joint works with Erwin Bolthausen and Yvan Velenik.
Schreiber Building Room 309 at 14:30.

Monday, May 16

Alexander Shamov, Weizmann Institute
Introduction to Gaussian multiplicative chaos
Abstract:
A subcritical Gaussian multiplicative chaos (GMC) is an "exponential" of a generalized (i.e. distributional) Gaussian field, normalized by its expectation. Assuming an appropriate zero-one law, (mixtures of) GMCs are essentially the only integrable random measures that are "local" functionals of the field. In the talk I will review general characterization and convergence results for subcritical GMCs and "logarithmicity"-type necessary conditions for their existence.
Schreiber Building Room 309 at 14:30.

Monday, May 23

Alexander Magazinov, Tel Aviv University
Short distance percolation of hard disks
Abstract:
In this talk I will focus on the standard construction of random point arrangements in the plane interacting by a simple hard core exclusion (or, equivalently, on the random packings of unit balls). The intensity parameter λ controls the density of the packing, similarly to that for the Poisson point process.
Given a packing of unit balls, replace each ball by its blow-up to a radius 1 + ε. The union of the blow-ups will be called the excluded volume. A natural question is whether the excluded volume has an infinite component, or, in other words, whether the packing percolates with parameter ε.
I will sketch the proof that a random packing percolates almost surely for every ε > 0 if the intensity λ is large enough, depending only on ε. This settles the problem posed in (Bowen, Lyons, Radin, Winkler, 2006) and extends the result of (Aristoff, 2014), where the case ε > 1/2 was solved.
Schreiber Building Room 309 at 14:30.

Double seminar! Note special time! Each talk will be 50 minutes.
Monday, May 30

Felix Pogorzelski, Technion
Naomi Feldheim, Stanford University
Pogorzelski title: Non-commutative quasicrystals
Pogorzelski Abstract:
The investigation of aperiodic point sets originates back to work of Meyer in the 70ies who pursued harmonic analysis on harmonious sets in Euclidean space. Shechtman's discovery of physical quasicrystals (1982) via X-ray experiments (diffraction) triggered a boom of the mathematical analysis of the arising scatter patterns. In recent work with Michael Björklund and Tobias Hartnick, we developed a spherical diffraction theory for cut-and-project sets in general lcsc groups, thus advancing into the non-commutative world. This seminar aims at describing these quasicrystals, as well as the dynamical systems which naturally arise from them. From here, we outline the essentials of a new spherical diffraction theory.
Feldheim title: Mean and Minimum
Feldheim Abstract:
Let X and Y be two unbounded positive independent random variables. Write Min_m for the probability of the event {min(X,Y) > m} and Mean_m for that of the event {(X+Y)/2 > m}. We show that the limit inferior of Min_m / Mean_m is always 0 (as m approaches infinity), regardless of the distributions of X and Y. We view this statement as a universal anti-concentration result, and discuss several implications. The proof is elementary but involved, relying on comparison to the "nearest" log-concave measure. We also provide a multiple-variables, weighted variant of this result in the i.i.d. case and pose a conjectured general result encompassing this phenomenon.
Joint work with Ohad Feldheim.
Schreiber Building Room 309 at 14:10.

Monday, June 6

Brandon Seward, Hebrew University of Jerusalem
The Ornstein isomorphism theorem for countably infinite groups
Abstract:
The well known Ornstein isomorphism theorem states that if two alphabets have the same Shannon entropy, then the corresponding Bernoulli shifts over the integers are isomorphic. Stepin proved that the class of groups satisfying this theorem is closed under extensions, and Ornstein and Weiss proved that all amenable groups satisfy this theorem. A few years ago Lewis Bowen made significant progress by proving that the isomorphism theorem holds for all countably infinite groups when one assumes that both alphabets have at least 3 letters. In this talk I will show that the isomorphism theorem holds for all countably infinite groups without any additional assumptions.
Schreiber Building Room 309 at 14:30.





Fall Semester 2015


Monday, October 19

Ryokichi Tanaka, Tohoku University
Random walks on hyperbolic groups: entropy and speed
Abstract:
We consider three fundamental quantities which are associated with random walks on groups: entropy, speed and volume growth (exponential growth rate of the group). The fundamental inequality due to Guivarc'h tells that the entropy does not exceed the speed times the volume growth. Vershik (2000) asked about the genuine equality case. We focus on hyperbolic groups, and characterize the equality case; namely, the equality holds if and only if the harmonic measure and a natural geometric measure--the Hausdorff measure (a Patterson-Sullivan measure)--on the boundary are equivalent. We also discuss open problems related to this question.
I will start with a history of the problem and mention recent progress. All notions will be explained in the talk.
Schreiber Building Room 309 at 14:30.

Monday, October 26

Asaf Nachmias, Tel Aviv University
Indistinguishability of trees in uniform spanning forests
Abstract:
The uniform spanning forest (USF) of an infinite connected graph G is the weak limit of the uniform spanning tree measure taken on exhausting finite subgraphs of G. It is easy to see that it is supported on spanning graphs of G with no cycles, but it need not be connected. Indeed, a classical result of Pemantle ('91) asserts that when G=Z^d, the USF is almost surely a connected tree if and only if d=1,2,3,4.
We prove that when G is a Cayley graph (or more generally, a unimodular random network) one cannot distinguish the connected components of the forest from each other by invariantly defined graph properties almost surely. This confirms a conjecture of Benjamini, Lyons, Peres and Schramm 2001.
Joint work with Tom Hutchcroft.
Schreiber Building Room 309 at 14:30.

Monday, November 2

Nishant Chandgotia, Tel Aviv University
Entropy Minimality and Four-Cycle Free Graphs
Abstract:
A topological dynamical system (X,T) is said to be entropy minimal if all closed T-invariant subsets of X have entropy strictly less than (X,T). In this talk we will discuss the entropy minimality of a class of topological dynamical systems which appear as the space of graph homomorphisms from Z^d to graphs without four cycles; for instance, we will see why the space of 3-colourings of Z^d is entropy minimal even though it does not have any of the nice topological mixing properties.
Schreiber Building Room 309 at 14:30.

Monday, November 9

Daniel Ueltschi, University of Warwick
The random interchange process on the hypercube
Abstract:
We study random permutations of the vertices of the hypercube. The permutations are given by products of (uniform, independent) random transpositions on edges. We establish the existence of a phase transition accompanied by cycles of diverging lengths. (Joint work with Roman Kotecký and Piotr Miłoś.)
Schreiber Building Room 309 at 14:30.

Monday, November 16

Vadim Gorin, MIT
Central Limit Theorem for discrete log-gases
Abstract:
A log-gas is an ensemble of N particles on the real line, for which the probability of a configuration is the power of the Vandermonde determinant times the product of a weight w(x) over the positions of particles. Such ensembles are widespread in the random matrix theory, while their discrete counterparts appear in numerous statistical mechanics models such as random tilings and last passage percolation, and also in the asymptotic representation theory. I will explain a new approach which gives Central Limit Theorems for global fluctuations of discrete log-gases for a wide class of the weights w(x). The approach is based on novel discrete equations, which are analogues of the loop equations known in the continuous settings.
Schreiber Building Room 309 at 14:30.

Monday, November 23

Yaar Solomon, Stony Brook University
The Danzer problem and a solution to a related problem of Gowers
Abstract:
Is there a point set Y in R^d, and C>0, such that every convex set of volume 1 contains at least one point of Y and at most C? This discrete geometry problem was posed by Gowers in 2000, and it is a special case of an open problem posed by Danzer in 1965. I will present two proofs that answers Gowers' question with a NO. The first approach is dynamical; we introduce a dynamical system and classify its minimal subsystems. This classification in particular yields the negative answer to Gowers' question. The second proof is direct and it has nice applications in combinatorics. The talk will be accessible to a general audience. [This is a joint work with Omri Solan and Barak Weiss].
Schreiber Building Room 309 at 14:30.

Monday, November 30

Itai Benjamini, Weizmann Institute
Coarse uniformization and percolation
Abstract:
We will present an elementary problem and a conjecture regarding percolation on planar graphs suggested by assuming quasi invariance of percolation crossing probabilities under coarse conformal uniformization.
Schreiber Building Room 309 at 14:30.

Monday, December 7

Rene Rühr, Tel Aviv University
Distribution of Primitive Points and Shapes of Lattices
Abstract:
Does the set of primitive vectors P(D) on large spheres of radius D in the d-dimensional Euclidean space (d>2) equidistribute when projected on the unit sphere, as D goes to infinity? A way to address this problem is by making use of homogeneous dynamics: To each set P(D), identify a closed orbit, and use recent measure rigidity results to show that these must equidistribute. Aka, Einsiedler and Shapira recently considered a refinement of this problem, attaching to each vector the shape of its orthogonal lattice, and showed joint equidistribution in the corresponding ambient space. For dimensions <6 some congruence conditions are assumed. In joint work with Manfred Einsiedler and Philipp Wirth, we give polynomial error rates in D and thereby removing congruence conditions for d=4,5. The focus of this talk will be on the transition to the dynamical problem, and why quantitative estimates are necessary to give the mere qualitative statement of equidistribution.
Schreiber Building Room 309 at 14:30.

Monday, December 14

Mira Shamis, Weizmann Institute
The supersymmetric formalism, applied to the density of states of random matrices
Abstract:
I will give an introduction to the supersymmetric formalism. Then I will show how it can be used to study the eigenvalue distribution of random matrices. The talk will be self-contained.
Schreiber Building Room 309 at 14:30.

Monday, December 21

Ron Rosenthal, ETH Zurich
Eigenvalue confinement and spectral gap for random simplicial complexes
Abstract:
We consider the adjacency operator of the Linial-Meshulam model for random simplicial complexes on n vertices, where each d-cell is added independently with probability p to the complete (d-1)-skeleton. From the point of view of random matrix theory, the adjacency matrix is a sparse, self adjoint random matrix with dependent entries. Under the assumption np(1-p) >> log^4 n, we prove that the spectral gap between the \binom{n-1}{d} smallest eigenvalues and the remaining \binom{n-1}{d-1} eigenvalues is np-2\sqrt{dnp(1-p)}(1+o(1)) with high probability. This estimate follows from a more general result on eigenvalue confinement. In addition, we prove that the global distribution of the eigenvalues is asymptotically given by the semicircle law. Based on a joint work with Antti Knowles.
Schreiber Building Room 309 at 14:30.

Monday, December 28

Igor Wigman, King's college London
On the number of nodal domains of toral eigenfunctions
Abstract:
This work is joint with Jerry Buckley.
We study the number of nodal domains of toral Laplace eigenfunctions. Following Nazarov-Sodin's results for random fields and Bourgain's de-randomisation procedure we establish a precise asymptotic result for ``generic" eigenfunctions. Our main results in particular imply an optimal lower bound for the number of nodal domains of generic toral eigenfunctions.
Schreiber Building Room 309 at 14:30.

Monday, January 4

Omri Sarig, Weizmann Institute
Measures of maximal entropy for surface diffeomorphisms (joint with Buzzi and Crovisier)
Abstract:
Newhouse showed that every C infinity diffeomorphism has at least one measure of maximal entropy. We show that in the case of surface diffeomorphisms, the number of such measures is finite, and in the topologically transitive case the measure is unique. This is joint work with Jerome Buzzi and Sylvain Crovisier.
Schreiber Building Room 309 at 14:30.

Monday, January 11

Wei Wu, New York University
Quantum triviality of a spin model on uniform spanning forests
Abstract:
Quantum triviality refers to the phenomenon that an interacting lattice model converges to a free field in the scaling limit. This has been established for Ising and Phi^4 models, at or above their upper critical dimension. We describe a simple spin model from uniform spanning forests in Z^d whose critical dimension is 4 and prove that the scaling limit is the bi-Laplacian Gaussian field for d≥ 4. At dimension 4, there is a logarithmic correction for the spin-spin correlation and the bi-Laplacian Gaussian field is a log correlated field. Based on joint works with Greg Lawler and Xin Sun.
Schreiber Building Room 309 at 14:30.





Special Summer Seminar 2015

Note special room and time!
Monday, July 20

Frank den Hollander, Leiden University
Breaking of ensemble equivalence in complex networks
Abstract:
It is generally believed that, for physical systems in the thermodynamic limit, the microcanonical description in terms of energy coincides with the canonical description in terms of temperature. However, various examples have been identified for which the microcanonical and canonical ensembles are not equivalent. A complete theory of this intriguing phenomenon is still missing.
In this talk we show that ensemble nonequivalence can manifest itself also in discrete enumeration problems. As specific examples, we consider ensembles of graphs with topological constraints. We show that, while graphs with a given number of links are ensemble-equivalent, graphs with a given degree sequence are not. This mathematical result provides a theoretical explanation for various ‘anomalies’ that have recently been observed in real-world networks.
While it is generally believed that ensemble nonequivalence is associated with long-range interactions, our findings show that it may naturally arise in systems with local constraints as well.
Joint work with Diego Garlaschelli, Joey de Mol, Andrea Roccaverde and Tiziano Squartini
Schreiber Building Room 209 at 14:10.





Spring Semester 2015


Monday, March 9

Omer Angel, University of British Columbia
Increasing subsequences in random walks
Abstract:
We consider the length of the longest increasing subsequence in an n step simple (and other) random walks, and show that the length is with high probability n^{1/2+o(1)}. Several related problems will be discussed. (with Richard Balka, Yuval Peres).
Schreiber Building Room 309 at 14:30.

Monday, March 16

Barak Weiss, Tel Aviv University
Everything is Illuminated (Except for at Most Finitely Many Points)
Abstract:
Suppose a light source is placed in a polygonal hall of mirrors (so light can bounce off the walls). Does every point in the room get illuminated? This elementary geometrical question was open from the 1950s until Tokarsky (1995) found an example of a polygonal room in which there are two points which do not illuminate each other. Resolving a conjecture of Hubert-Schmoll-Troubetzkoy, in joint work with Lelievre and Monteil we prove that if the angles between walls is rational, every point illuminates all but at most finitely many other points. The proof is based on recent work by Eskin, Mirzakhani and Mohammadi in the ergodic theory of the SL(2,R) action on the moduli space of translation surfaces. The talk will serve as a gentle introduction to the amazing results of Eskin, Mirzakhani and Mohammadi.
Schreiber Building Room 309 at 14:30.

Monday, March 23

Tom Hutchcroft, University of British Columbia
Random Hyperbolic Triangulations: Circle Packing and Random Walk
Abstract:
For bounded degree planar graphs, a rich theory has been developed connecting the behaviour of random walk to the geometry of the circle packing embedding. In this talk, I will develop a parallel theory for random triangulations, without the assumption of bounded degree.
First, I will show that the circle packing type (hyperbolic or parabolic) is determined by the average degree of the triangulation, obtaining a new proof of the Benjamin-Schramm Recurrence Theorem.
Secondly, in the hyperbolic case, I will discuss the limiting behaviour of the random walk, showing that the geometric boundary given by the circle packing may be identified with the Poisson boundary of the graph.
Joint work with Omer Angel, Asaf Nachmias and Gourab Ray.
Schreiber Building Room 309 at 14:30.

Monday, March 30 and Monday, April 6

Passover break - No seminar.

Monday, April 13

Gidi Amir, Bar-Ilan University
Amenability, entropy and the Liouville property for automaton groups via random walks and electrical networks
Abstract:
Automaton groups are groups of automorphisms of regular rooted trees that have certain self-similar properties. These include important examples such as the Grigorchuk group, the basilica group and many others. In this talk we will study the electrical resistance between vertices in some Schreier graphs of these groups and show how these can be used to prove amenability of low activity automaton groups (degree ≤2), as well as deduce the Liouville property and uniform estimates for the entropy of (symmetric, finitely supported) random walks on bounded degree automaton groups.
No prior knowledge of automaton groups, amenability or the Liouville property will be assumed.
This talk is based on joint works with Omer Angel, Nicolas Matte Bon and Balint Virag.
Schreiber Building Room 309 at 14:30.

Monday, April 20

Benjamin Weiss, The Hebrew University of Jerusalem
Weak mixing properties for non-singular actions
Abstract:
For a probability measure preserving action of a locally compact group G, there are various characterizations of weak mixing. Analogous definitions can be given when the action merely preserves the measure class and I will discuss some implications that hold between these properties (based on joint work with Eli Glasner)
Schreiber Building Room 309 at 14:30.

Monday, April 27

Chaim Even Zohar, Hebrew University
Invariants of Random Knots
Abstract:
Random curves in space and how they are knotted give an insight into the behavior of "typical" knots and links. They have been studied by biologists and physicists in the context of the structure of random polymers. There have been many results obtained via computational experiment, but few explicit computations.
In work with Hass, Linial and Nowik, we study random knots based on petal projections, developed by Adams et al. (2012). We have found explicit formulas for the distribution of the linking number of a two component link. We also find formulas for the moments of two finite type invariants of knots, the Casson invariant and another coefficient of the Jones polynomial.
No background in Knot Theory will be supposed. All terms above will be explained. Joint work with Joel Hass, Nati Linial, and Tahl Nowik.
Schreiber Building Room 309 at 14:30.

Monday, May 4

Mira Shamis, Weizmann Institue
Wegner estimates for deformed Gaussian ensembles, and the Wegner orbital model
Abstract:
The deformed Gaussian Orthogonal Ensemble is obtained by adding a deterministic real symmetric matrix to a random matrix drawn from the Gaussian Orthogonal Ensemble. We shall discuss the behavior of the inverse X to such a random matrix, and in particular find the sharp magnitude and tails of its norm ||X||, and of the norm ||X u|| for a deterministic vector u.
We discuss applications to the Wegner orbital model, a random operator related to the Anderson model which describes a quantum particle with many internal degrees of freedom (orbitals) in a disordered medium. In particular, we establish localization at strong disorder, with sharp dependence of the localization threshold on the number of orbitals, and prove a uniform bound on the density of states.
Joint work with M. Aizenman, R. Peled, J. Schenker, S. Sodin.
Schreiber Building Room 309 at 14:30.

Monday, May 11

David Gilat, Tel Aviv University
Convergence in Distribution (D), Convergence in Probability (P) and Almost-Sure convergence (AS) of Martingales. Old Stuff with a New Twist
Abstract
Schreiber Building Room 309 at 14:30.

Note special time!
Monday, May 18

Jonathan Hermon, UC Berkeley
Characterization of cutoff for reversible Markov chains
Abstract:
A sequence of Markov chains is said to exhibit (total variation) cutoff if the convergence to stationarity in total variation distance is abrupt. We consider reversible lazy chains. We prove a necessary and sufficient condition for the occurrence of the cutoff phenomena in terms of concentration of hitting time of "worst" (in some sense) sets of stationary measure at least alpha, for some 0 < alpha < 1.
We also give general bounds on the total variation distance of a reversible chain at time t in terms of the probability that some "worst" set of stationary measure at least alpha was not hit by time t. As an application of our techniques we show that a sequence of lazy Markov chains on finite trees exhibits a cutoff iff the product of their spectral gaps and their (lazy) mixing-times tends to infinity.
Joint work with Riddhipratim Basu and Yuval Peres.
Schreiber Building Room 309 at 14:20.

Monday, May 25

Margherita Disertori, Universität Bonn
Some results on history dependent stochastic processes
Abstract:
Edge reinforced random walk (ERRW) and vertex reinforced jump processes (VRJP) are history dependent stochastic processes, where the particle tends to come back more often on sites it has already visited in the past. For a particular scheme of reinforcement these processes are random walks in random environment (mixing of reversible Markov chains) whose mixing measure can be related to a non-linear sigma model introduced in the context of random matrix models for quantum diffusion. I will give an overview on these models and explain some recent results.
Schreiber Building Room 309 at 14:30.

Monday, June 1

No seminar due to many participants being away.
Audience is recommended to attend Dembo's colloquium talk and Solomyak's analysis seminar talk.

Monday, June 8

Yuval Peres, Microsoft Research
Using random walks to analyze Prediction with Expert Advice
Abstract:
We study the classical problem of prediction with expert advice in the adversarial setting with a geometric stopping time. Cover (1965) gave the optimal algorithm that minimizes worst-case regret for the case of 2 experts. In this talk, I will describe the optimal algorithm, adversary and regret for the case of 3 experts. We will see that optimal algorithm for 2 and 3 experts is a probability matching algorithm (analogous to Thompson sampling) against a particular randomized adversary. Remarkably, it turns out that this algorithm is not only optimal against this adversary, but also minimax optimal against all possible adversaries. The analysis of the optimal adversary relies on delicate random walk estimates. At the end of the talk, I will discuss the case of "Bandit feedback", when we just learn the gain of the action we chose, and analyze the effects of imposing a switching cost. This analysis uses a Gaussian Branching random walk.
(Talk based on joint works with Nick Gravin and Balu Sivan and with Ofer Dekel, Jian Ding and Tomer Koren.)
Schreiber Building Room 309 at 14:30.

Monday, June 15

Ohad Feldheim, University of Minnesota
Long-range order in random three colourings of Zd
Abstract:
Consider a random colouring of a bounded domain in Zd with the probability of each colour configuration proportional to exp(-β*N(F)), where β>0 is a parameter (representing the inverse temperature) and N(F) is the number of nearest neighbouring pairs coloured by the same colour. This is the anti-ferromagnetic 3-state Potts model of statistical physics, used to describe magnetic interactions in a spin system. The Kotecký conjecture is that in such a model, for d≥3 and high enough β, a sampled colouring will typically exhibit long-range order, placing the same colour at most of either the even or odd vertices of the box. We give the first rigorous proof of this fact for large d. This extends previous works of Peled and of Galvin, Kahn, Randall and Sorkin, who treated the case β equals infinity.
The main ingredient in our proof is a new structure theorem for 3-colourings which characterises the ways in which different "phases" may interact, putting special emphasis on the role of edges connecting vertices of the same colour. We also discuss several related conjectures. No background in statistical physics will be assumed and all terms will be explained thoroughly.
Joint work with Yinon Spinka.
Schreiber Building Room 309 at 14:30.





Fall Semester 2014


Monday, October 27

Uri Shapira, Technion
Escape of mass for measures invariant under the diagonal group
Abstract:
I will discuss some observations regarding the topology of the space of A-invariant probability measures on the space of lattices (A being the diagonal group in SL_n). In particular, I will explain a construction of Cassels giving rise to a sequence of A-invariant ergodic probability measures (supported on compact A-orbits) which converge to the zero measure (i.e. full escape of mass).
Schreiber Building Room 309 at 14:30.

Monday, November 3

Victor Kleptsyn, Institut de Recherche Mathématique de Rennes
Towards rigorous construction for random metrics : the cut-off process
Abstract:
One of the open problems in the domain of quantum gravity is the one of constructing a random metric on a manifold as a limit of a multiplicative cascade; if constructed for the case of a square, it can be thought as the realization of 'exp(DGFF) |dz|’.
Though this problem is well-known, there are very few rigorous known results. One of them is the work of Benjamini and Schramm for the multiplicative cascades on the interval, where the sequence of distances forms a martingale. The (martingale-related) convergence of measures is a key element in a work of Duplantier and Sheffield on the KPZ formula. Finally, the results of Le Gall and Miermont show that one can consider a random metric on the sphere as a limit of random planar maps.
The main result of our work is the rigorous construction of a random metric via multiplicative cascades on hierarchical graphs (like Benjamini's eight graph, dihedral hierarchical lattice, etc.), this situation being both still accessible due to the graph structure, but already complicated due to the high non-uniqueness of candidates for geodesic lines.
A key argument, that allows to find a stationary law for the glueing process, is the cut-off process: instead of looking for a critical renormalization value, we 'stabilize' the process in the supercritical regime by adding a 'shortcut', and then pass to the 'diagonal' limit (the renormalization parameter tends to the critical value, and at the same time the influence of the shortcut tends to zero).
Joint work with M. Khristoforov and M. Triestino.
Schreiber Building Room 309 at 14:30.

Monday, November 10

Antal Jarai, University of Bath
Sandpiles and the one-end property of uniform spanning forests
Abstract:
The Abelian sandpile model is a stochastic cellular automaton introduced by physicists Bak, Tang and Wiesenfeld (1987) and Dhar (1990). The recurrent states of the model are in bijection with spanning trees of the underlying graph; due to Majumdar and Dhar (1992). This bijection has been very fruitful, as it allows one to translate sandpile problems into questions about uniform spanning trees. When one considers the limit of infinite graphs, an important role is played by the condition that each component of the wired uniform spanning forest has one end; a question studied by Pemantle (1991), Benjamini, Lyons, Peres and Schramm (2001), and Lyons, Morris and Schramm (2008). I will illustrate the above connection in two recent results: i) the bijection is extended to infinite graphs in a way that preserves the corresponsing probability measures (joint with S. Gamlin); ii) the avalanche exponent is studied for sandpiles on supercritical Galton-Watson trees (joint with W. Ruszel and E. Saada).
Schreiber Building Room 309 at 14:30.

Monday, November 17

Gady Kozma, Weizmann Institute
Linearly reinforced random walk
Abstract:
We consider a walker which changes its environment as it goes, strengthening each edge it crosses so that it is more likely to cross it again the next time it meets it. The particular case where the reinforcement is linear has a lot of extra structure related to exchangeability and supersymmetry, and enjoys a phase transition in the strength of the reinforcement. We show that when the reinforcement is strong enough the walk enters a condensed phase. Joint work with Angel and Crawford.
Schreiber Building Room 309 at 14:30.

Monday, November 24

Omri Sarig, Weizmann Institute
Ergodic properties of the measure of maximal of entropy for smooth three dimensional flows
Abstract:
Geodesic flows on compact connected surfaces of non-positive, non-identically zero curvature are Bernoulli with respect to the Liouville measure (Ornstein-Weiss, Ratner, Pesin). We show that this is the case for the measure of maximal entropy. Some of our results hold for all smooth three dimensional flows.
Joint ongoing work with Ledrappier and Lima.
Schreiber Building Room 309 at 14:30.

Monday, December 1

Eitan Bachmat, Ben-Gurion University
A geometric Pollaczek-Khinchine formula involving increasing subsequences, modular transformations and optics
Abstract:
Recently, a couple of airlines experimented with the following boarding policy. Passengers that do not have overhead bin luggage board before passengers who do. In the talk we will analyze this and related policies using the notions presented in the title.
Schreiber Building Room 309 at 14:30.

Monday, December 8

Kiran Parkhe, Technion
Groups of polynomial growth and 1D dynamics
Abstract:
Let M be a connected one-manifold, and G a group of homeomorphisms of M which is finitely-generated and virtually nilpotent, i.e., which has polynomial growth. We prove a structure theorem which says, roughly, that the manifold decomposes into wandering regions (in which no G-orbit is dense), and minimal regions (in which every G-orbit is dense); and on the latter, the action is actually Abelian.
As a corollary, if G is a group of polynomial growth of degree d, then for any alpha < 1/d, any continuous G-action on M is conjugate to an action by C^{1 + alpha} diffeomorphisms. This strengthens a result of Farb and Franks.
Schreiber Building Room 309 at 14:30.

Monday, December 15

Daniel Ueltschi, Warwick
Random loop models and quantum spin systems
Abstract:
The random loop representations of the quantum Heisenberg models allow to study these systems using probabilistic methods. They were introduced twenty years ago by Toth and Aizenman-Nachtergaele. I will present some rigorous results about a phase transition with long loops in hypercubes (joint work with R. Kotecky and P. Milos), and about the decay of certain quantum correlations (joint work with J. Bjornberg).
Schreiber Building Room 309 at 14:30.

Monday, December 22

Michael Krivelevich, Tel Aviv University
The Phase Transition in Site Percolation on Pseudo-Random Graphs
Abstract:
We establish the existence of the phase transition in site percolation on pseudo-random d-regular graphs. Let G=(V,E) be an (n,d,lambda)-graph, that is, a d-regular graph on n vertices in which all eigenvalues of the adjacency matrix, but the first one, are at most lambda in their absolute values. Form a random subset R of V by putting every vertex v in V into R independently with probability p. Then for any small enough constant epsilon>0, if p=(1-epsilon)/d, then with high probability all connected components of the subgraph of G induced by R are of size at most logarithmic in n, while for p=(1+epsilon)/d, if the eigenvalue ratio lambda/d is small enough as a function of epsilon, then typically R contains a connected component of size at least epsilon n/d and a path of length proportional to epsilon^2n/d.
Schreiber Building Room 309 at 14:30.

Double seminar! Note special time! Each talk will be 50 minutes.
Monday, December 29

Menny Aka, ETH Zurich
Matan Harel, Université de Genève
Aka title: Diophantine approximation on submanifolds of real matrices and Lie groups
AKA Abstract:
I will explain how homogeneous dynamics can help to understand Diophantine properties on submanifolds of matrices and on Lie groups. In particular, I'll present notions and results from http://arxiv.org/abs/1307.1489 and http://arxiv.org/abs/1410.3996.
The first answers the question of Diophanticity of nilpotent Lie groups and in particular establishes the first examples of non-Diophantine Lie groups. The second paper aims to compute the critical exponent of Nilpotent Lie groups. To this end we study the Diophantine exponent of analytic submanifolds of mxn real matrices, answering a set of questions of Beresnevich, Kleinbock and Margulis.
Harel title: The Localization Phase Transition in Random Geometric Graphs with Too Many Edges
Harel Abstract:
Consider the Gilbert random geometric graph G(n, r(n)), given by a connecting two points of a Poisson Point Process of intensity n on the unit torus whenever their distance is smaller than the parameter r(n). This model is conditioned on the rare event that the number of edges observed, |E|, is greater than [1 + delta (n)] times its expectation. We show that, when delta is fixed or vanishing sufficiently slowly in n, there exists a "giant clique" with almost all the excess edges forced into the model by the conditioning event. If delta vanishes sufficiently quickly, the largest clique will be, at most, a constant multiple of the usual clique number. Finally, we discuss progress in finding a phase transition function delta_(0)(n), so that when delta is much bigger than delta_(0), the giant clique scenario holds, while delta much smaller than delta_(0) implies no giant clique.
Schreiber Building Room 309 at 14:10.

Monday, January 5

Anish Ghosh, Tata Institute of Fundamental Research
Diophantine approximation on varieties
Abstract:
I will discuss the problem of Diophantine approximation on homogeneous varieties of semisimple groups and present analogues of results in classical Diophantine approximation. In particular, I will discuss the problem of counting solutions for Diophantine inequalities on varieties. This is joint work with Alex Gorodnik and Amos Nevo.
Schreiber Building Room 309 at 14:30.

Monday, January 12

In-Jee Jeong, Princeton University
Dyadic models in Fluid Dynamics
Abstract:
Dynamics of fluids are mathematically described by Euler and Navier-Stokes equations. Among many interesting questions regarding these equations, I will describe Kolmogorov's scaling theory and Onsager's conjecture. Then I will describe a toy model for Euler equations, which is an infinite system of simple ODEs. Analogues of Kolmogorov theory and Onsager conjecture will be proved in this toy model.
Schreiber Building Room 309 at 14:30.

Monday, January 19

Ofer Zeitouni, Weizmann Institute
Controlled martingales and hitting probabilities
Abstract:
Consider an integer valued martingale S_n=sum_{i=1}^n X_i with bounded step size, with E(X_i^2 | F_{i-1}) ≥ δ > 0. Is it true that P(|S_n|≤1) ≤ C(δ)/sqrt{n}?
We will explain the (negative) answer to this question and derive sharp upper and lower bounds through a stochastic control formulation.
Based on joint works with Ori Gurel-Gurevich and Yuval Peres, and with Scott Armstrong
Schreiber Building Room 309 at 14:30.





Special Summer Seminar 2014

Monday, July 28

Eviatar B. Procaccia, University of California, Los Angeles
Quenched invariance principle for simple random walk on clusters in correlated percolation models
Abstract:
Quenched invariance principle and heat kernel bounds for random walks on infinite percolation clusters and among i.i.d. random conductances in Z^d were proved during the last two decades.The proofs of these results strongly rely on the i.i.d structure of the models and some stochastic domination with respect to super-critical Bernoulli percolation. Many important models in probability theory and in statistical mechanics, in particular, models which come from real world phenomena, exhibit long range correlations. In this talk I will present a new quenched invariance principle, for simple random walk on the unique infinite percolation cluster for a general class of percolation models on Z^d, d≥22, with long-range correlations. This gives new results for random interlacements in dimension d≥3 at every level, as well as for the vacant set of random interlacements and the level sets of the Gaussian free field in the regime of the so-called local uniqueness (which is believed to coincide with the whole supercritical regime). An essential ingredient of the proof is a new isoperimetric inequality for correlated percolation models.
Joint work with Ron Rosenthal and Artem Sapozhnikov.
Schreiber Building Room 209 at 14:30.





Spring Semester 2014


Monday, February 17

Elliot Paquette, Weizmann Institute
Choices and Intervals
Abstract:
Consider the following point process on the unit circle. Finitely many distinct points are placed on the circle in any arbitrary configuration. This configuration of points subdivides the circle into a finite number of intervals. At each time step, two points are sampled uniformly from the circle. Each of these points lands within some pair of intervals formed by the previous configuration. Add the point that falls in the larger interval to the existing configuration of points, discard the other, and then repeat this process.
We will study the behavior of a typical interval, and we will show that as the number of points tends to infinity, this has an almost sure limit, which we characterize. The convergence is established by showing that the size-biased empirical distribution evolves in the limit according to a certain deterministic evolution equation. Although this equation involves a non-local, non-linear operator, it can be studied thanks to a carefully chosen norm with respect to which this operator is contractive. To show the convergence in the presence of noise, we adapt a method of Kushner and Clark to the infinite-dimensional setting.
This is joint work with Pascal Maillard (Weizmann).
Schreiber Building Room 309 at 14:30.

Double seminar! Note special time! Each talk will be 50 minutes.
Monday, February 24

Pascal Maillard, Weizmann Institute
Luca Marchese, Université Paris 13
Maillard title: Contraction of trees
Maillard Abstract:
Take a random rooted tree and contract each edge with probability p, where contracting an edge means removing it from the tree and identifying its head and tail. Which random trees are invariant (in law) under this transformation? I will present recent results (joint with Olivier He'nard) which characterize all one-ended trees invariant under this operation, and under the more general operation where edges on the infinite ray are contracted with a possibly different probability q. I will also describe the relationship with (real-valued) self-similar processes and quasi-stationary distributions of linear pure death processes, as well as the common aspects and differences with other renormalization procedures of trees or graphs.
Marchese title: Lagrange spectra for translation surfaces via renormalization
Marchese Abstract:
We introduce Lagrange spectra of closed-invariant loci for the action of SL(2,R) on the moduli space of translation surfaces, generalizing the classical Lagrange Spectrum. We treat basic topological issues of spectra, like closure, density of dynamically relevant subsets and existence of an Hall ray. Our approach is renormalization: we get explicit formulas in terms of different continued fraction algorithms. As a consequence, we prove the equivalence of several definitions of bounded Teichmüller geodesics and bounded type interval exchange transformations and moreover we get quantitative estimates for the excursions to the boundary of moduli space in terms of norms of positive matrices in the Rauzy-Veech induction. Joint work with Pascal Hubert and Corinna Ulcigrai.
Schreiber Building Room 309 at 14:10.

Monday, March 3

Yinon Spinka, Tel Aviv University
The loop O(n) model
Abstract:
A loop configuration on the hexagonal (honeycomb) lattice is a finite subgraph of the lattice in which every vertex has degree 0 or 2, so that every connected component is a cycle. The loop O(n) model on the hexagonal lattice is a probability measure on loop configurations, in which the probability of a configuration is proportional to x^(#edges) n^(#loops) and x,n>0 are parameters called the edge and loop weights. This model arises in statistical physics and is related to the Ising model and self-avoiding walks as well as to the so-called XY and Heisenberg models. We study the loop O(n) model for large values of the loop weight n. We prove that the model undergoes a transition from a disordered to an ordered phase as the edge weight x varies. However, we show that for any value of the edge weight, the probability that the origin is surrounded by a loop of length k decays exponentially in k.
No prior knowledge in statistical mechanics will be assumed and all notions will be explained. Joint work with Hugo Duminil-Copin, Ron Peled and Wojciech Samotij.
Schreiber Building Room 309 at 14:30.

Monday, March 10

Mike Hochman, The Hebrew University of Jerusalem
Dimension of self-similar sets and additive combinatorics
Abstract:
I'll present recent progress on the dimension of self-similar sets and measures on the line in the presence of nontrivial "overlaps" in the construction. In particular this resolves a problem of Furstenberg on sums of dilations of certain Cantor sets, and gives information about many other conjectures, e.g. Bernoulli convolutions and the Keane-Smorodinsky {0,1,3} problem. The key new ingredient is to introduce methods from additive combinatorics.
Schreiber Building Room 309 at 14:30.

Monday, March 17

Boris Hanin, Northwestern University
Nodal Sets of Random Isotropic Hermite Functions
Abstract:
Random isotropic Hermite functions of fixed degree in R^d have an SO(d−1) symmetry and are in some ways analogous to random spherical harmonics of fixed degree on S^d, whose nodal sets have been the subject of many recent studies. However, there is a fundamentally new aspect to this ensemble, namely the existence of allowed and forbidden regions. In the allowed region, the Hermite functions behave like spherical harmonics, while in the forbidden region, Hermite functions are exponentially decaying and it is unclear to what extent they oscillate and have zeros.
The purpose of this talk is to present a new result about the expected volume of the zero set of a random Hermite function in both the allowed and forbidden regions.
This is joint work with Steve Zelditch and Peng Zhou.
Schreiber Building Room 309 at 14:30.

Monday, March 24

Idan Perl, Ben-Gurion University
Extinction window of mean field branching annihilating random walk
Abstract:
We study a model of growing population that competes for resources. At each time step, all existing particles reproduce and the offspring randomly move to neighboring sites. Then at any site with more than one offspring the particles are annihilated. This is a non-monotone model, which makes the analysis more difficult. We consider the extinction window of this model in the finite mean-field case, where there are n sites but movement is allowed to any site (the complete graph). We show that although the system survives for exponential time, the extinction window is logarithmic.
Schreiber Building Room 309 at 14:30.

Double seminar! Note special time! Each talk will be 50 minutes.

Monday, March 31

Nick Travers, Technion
Ioan Manolescu, Université de Genève
Travers title: Inversion statistics and longest increasing subsequence for k-card-minimum random permutations
Travers Abstract:
The k-card-minimum procedure for generating a random permutation of [n] = {1,...,n} is defined as follows. Begin with a deck of n cards labeled 1,...,n and n initially empty positions on a table labeled 1,...,n from left to right. Then, at each time t = 1,...,n choose independently and uniformly at random k cards from the remaining n - t + 1 cards in the deck, remove the lowest numbered (minimum) of these k cards, and place it on the table in position t. The independent sampling is done with replacement, so that the k cards chosen at each time t are not necessarily all distinct.
We prove a weak law of large numbers and central limit theorem for the number of inversions in a random permutation generated according to this procedure, both with fixed k and when k grows sublinearly in n. We also establish the rate of growth of the longest increasing subsequence in a random permutation generated according to this procedure when k grows as a sublinear power of n.
Manolescu title: Planar lattices do not recover from forest fires
Manolescu Abstract:
Self-destructive percolation with parameters p, delta is obtained by taking a site percolation configuration with parameter p, closing all sites belonging to the infinite cluster, then opening every site with probability delta, independently of the rest. Call theta(p,delta) the probability that the origin is in an infinite cluster in the configuration thus obtained. For two dimensional lattices, we show the existence of delta > 0 such that, for any p > p_c , theta(p,delta) = 0. This proves a conjecture of van den Berg and Brouwer, who introduced the model. Our results also imply the non-existence of the infinite parameter forest-fire model.
Schreiber Building Room 309 at 14:10.

Monday, April 7

Michael Keane, Wesleyan University
The binomial transformation
Abstract:
In recent years, interest has developed in a special measure preserving transformation, which I call the binomial transformation. In this lecture, I'd like to present two related definitions, and then explain a simple proof that the binomial transformation is ergodic. This proof extends to a larger subclass of adic transformations. It has been conjectured that the binomial transformation is weakly mixing, but no proof of this is known to me.
Schreiber Building Room 309 at 14:30.

Monday, April 14

Passover break - No seminar.

Monday, April 21

Passover break - No seminar.

Monday, April 28

Adela Svejda, Technion
Clock processes on infinite graphs and aging in Bouchaud's asymmetric trap model
Abstract:
A clock process is the total time that elapses along a given length of the trajectory of a random motion. It is the key object in connection with aging - a phenomenon of random dynamics in random environments whose convergence towards equilibrium becomes increasingly slower. Based on a method due to Durrett and Resnick, Bovier and Gayrard established convergence criteria for clock processes for dynamics on finite graphs. Based on the same method we study clock processes of dynamics in random environments on infinite graphs. As an application we prove that Bouchaud's asymmetric trap model on Z^d, d≥2, ages. This talk is based on joint work with V. Gayrard.
Schreiber Building Room 309 at 14:30.

Monday, May 5

Memorial day - No seminar.

Monday, May 12

Alexey Gladkich, Tel Aviv University
The cycle structure in random Mallows permutation
Abstract:
The Mallows model is a probability measure on permutations in S_n in which the probability of a permutation pi is proportional to q^{inv(pi)}, where inv(pi) denotes the number of inversions in pi and 0<q<1 is a parameter of the model. The model is an example of a class of distributions called spatial random permutations in which the distribution is biased to be close to the identity in a certain underlying geometry. We study the cycle structure of a permutation sampled from the Mallows model, partially addressing a question of Borodin, Diaconis and Fulman. Our main result is that the expected length of the cycle containing a given point is of order min(1/(1-q)^2, n). In contrast, the expected length of a uniformly chosen cycle is of order min(1/(1-q),n / log(n)). We also overview some related results and conjectures about spatial random permutations. No prior knowledge of random permutations will be assumed. Joint work with Ron Peled.
Schreiber Building Room 309 at 14:30.

Monday, May 19

Ronggang Shi, Tel Aviv University
Pointwise equidistribution for one parameter diagonalizable group action on homogenous space
Abstract:
Let Γ be a lattice in a noncompact simple Lie group L⊂GL_n. Suppose that {g_t} is a nontrivial one parameter diagonalizable subgroup of L. For certain proper subgroup U of the unstable horospherical subgroup of {g_t} we show that for any x∈L/Γ the trajectory {g_t ux: 0≤ t≤ T} is uniformly distributed with respect to the probability Haar measure of L/Γ as T tends to infinity, for almost every u in U.
Note special room!
Schreiber Building Room 006 at 14:30.

Monday, May 26

Zakhar Kabluchko, Universität Ulm
Complex zeros of random partition functions
Abstract:
Let X_1,X_2,... be independent standard normal random variables. The partition function of Derrida's Random Energy Model (REM) at an inverse temperature β is defined as Z_n(β) = ∑k=1..2^n exp(β sqrt(n) X_k). We study the fluctuations of the random variable Z_n(β) for complex values of β and describe the structure of complex zeros of the random analytic function Z_n as n tends to infinity. The local structure of zeros is described in terms of two interesting objects: the plane Gaussian Analytic Function and a zeta function associated to the Poisson process. We will also study the Generalized Random Energy Model (GREM), a generalization of the Random Energy Model involving long-range correlations between the energies. The phase diagram of the GREM in the complex plane and the distribution of the complex zeros of its partition function will be described. This is joint work with Anton Klimovsky.
Schreiber Building Room 309 at 14:30.

Monday, June 2

Jacob Kagan, Weizmann Institute
A translation invariant IDLA model
Abstract:
We consider a kind of 2D, directed, tree growing process starting at the line Z. Growth is excluding in the sense that a tree cannot grow further when another tree blocks its way. The question under consideration is whether there exists an infinite tree or not, and we prove the latter.
Ergodicity gives right away that there are either no infinite trees or a positive density of infinite trees on the starting line. The argument then proceeds by coupling the trees with a certain first passage percolation and then deducing that infinite trees do not occur almost surely.
This is a joint work with Noam Berger and Eviatar Procaccia.
Schreiber Building Room 309 at 14:30.

Monday, June 9

Eliran Subag, Weizmann Institute
Extremal point processes and freezing
Abstract:
It has been recently shown that the extremal point processes of the branching Brownian motion (BBM), 2-speed BBM, branching random walk, and extremes of the 2D Gaussian free field have the structure of a shifted decorated Poisson point process (SDPPP). On the other hand, some of those and other related models have been shown to exhibit `freezing' - a phenomenon related to the Laplace functional, which is mostly discussed in physics literature. We introduce a generalized notion of freezing and show that, under some technical assumptions, it characterizes the structure of SDPPP.
Joint work with Ofer Zeitouni.
Schreiber Building Room 309 at 14:30.





Fall Semester 2013


Monday, October 14

Isaac Meilijson, Tel Aviv University
Placing multiple chips in the roulette table
Abstract:
In Dynamic Programming, mixed strategies consist of randomizing the choice of actions. In some problems, such as portfolio management, it makes sense to diversify actions rather than choosing among them purely or randomly. Optimal betting in casinos and roulette by a gambler with fixed goal was studied by Dubins & Savage 1965 and their school without the element of diversification (betting simultaneously on different holes of the roulette), once it was proved (Smith's theorem - Smith 1967, Dubins 1972 and Gilat & Weiss 1976) that diversification doesn't increase the probability of reaching the goal. The scope of this finding is questioned, as it was based on the assumption that the holes on which gamblers can bet are disjoint, such as 1 and BLACK in regular roulette. A counterexample is provided in which holes are nested, such as 1 and RED. Thus, it may be rational for gamblers with fixed goal to place chips on more than one hole at the table.
Schreiber Building Room 309 at 14:30.

Monday, October 21

Nick Crawford, Technion
Large Deviation Estimates for Emptiness Formation
Abstract:
In this talk I will introduce a class of equilibrium quantum systems, called Anisotropic Heisenberg Models, on the d-dimensional integer lattice. In the quantum setting, the Hamiltonian is an operator (rather than a function on phase space). Nevertheless, in 1 dimension in particular and by varying a certain parameter in the definition, it is possible (through path integral expansions) to connect this family of models to a number of more conventional classical particle systems and statistical mechanics models, including the 6 vertex model, self dual 4 state Potts model, Dyson random walk, and the random stirring model.
In finite volume, it is possible to view (the amplitude squared of) the eigenfunctions of the Hamiltonian operator as probability distributions over the collection of all subsets, or configurations of particles, of the fixed volume of interest. The question we study concerns certain "large deviation events" of the ground-state (ie minimal eigenvalue) eigenfunctions. In particular we show, in the thermodynamic limit, that the probability of having no particles in a box of side-length L around the origin decays with L as exp(-cL^{d+1}) (upper and lower bounds). This signifies a very strong correlation structure in the underlying probability measures reminiscent of and connected to large deviations for eigenvalue spacings in GUE.
Joint work with Stephen Ng and Shannon Starr
Schreiber Building Room 309 at 14:30.

Monday, October 28

Alexander Gorodnik, University of Bristol
Diophantine approximation by orbits
Abstract:
The classical theory of Diophantine approximation quantifies the density of rational number in the real line. In a joint work with A. Ghosh and A. Nevo we consider an analogous problem of approximating by dense orbits for actions on homogeneous spaces. We explain a general approach which allows to establish quantitative density and gives the best possible exponents of approximation in a number of cases.
Schreiber Building Room 309 at 14:30.

Monday, November 4

Corinna Ulcigrai, University of Bristol
Ergodic properties of the Ehrenfest model and other infinite periodic billiards
Abstract:
The Ehrenfest model, introduced in 1912 as a mathematical physics model, is a planar billiard in which a particle moves without friction bouncing elastically at periodically spaced rectangular scatterers. Almost no rigorous results on this model were known until very recently, when several breakthroughs on its recurrence, diffusion and ergodicity properties were achieved, thanks with the connection with the study of periodic translation surfaces.
In this talk, after briefly surveying the known results, we focus on ergodicity. In joint work with K. Fraczek, we show that for almost every direction the Ehrenfest model and other infinite periodic billiards are not ergodic.
Schreiber Building Room 309 at 14:30.

Monday, November 11

Omer Tamuz, Microsoft Research and MIT
Stabilizer Rigidity in Irreducible Group Actions
Abstract:
An Invariant Random Subgroup (IRS) is a subgroup-valued random variable that is invariant to conjugation. IRSs are probabilistic generalizations of normal subgroups, and share many of their properties. IRSs arise naturally as the stabilizers of measure preserving actions.
In this talk we will show that all irreducible IRSs of product groups, and of higher rank Lie groups, are co-amenable in some normal subgroup. This implies generalizations and strengthenings of similar theorems by Stuck-Zimmer and Bader-Shalom.
Joint work with Yair Hartman.
Schreiber Building Room 309 at 14:30.

Monday, November 18

Naomi Feldheim, Tel Aviv University
Long gaps between sign-changes of stationary Gaussian processes
Abstract:
The probability of N independent Gaussians to be all positive is 2^{-N}. In many simple examples, the probability of N stationary Gaussians to be positive is shown to be bounded between exponents in N, thus demonstrating "independent-like" behavior. Which infinite Gaussian sequences with stationary distribution have this property? How does it relate to their correlations?
Together with Ohad Feldheim, we give a partial answer by presenting broad sufficient conditions for upper and lower exponential bounds on the probability in question. The results hold also for Gaussian stationary functions. Particular cases of this question were addressed by Newell and Rosenblatt in the 1960's and by Antezana, Buckley, Marzo and Olsen in 2012.
Schreiber Building Room 309 at 14:30.

Monday, November 25

Roman Kotecký, University of Warwick and Charles University
Long range order for random colourings on planar lattices
Abstract:
We establish a phase transition (entropic long range order) for the uniform random perfect 3-colourings on a class of planar quasi-transitive graphs. The proof is based on an enhanced Peierls argument (which is of independent interest even for the Ising model for which it extends the range of temperatures with proven long range order) combined with an additional percolation argument. The motivation stemming from Potts antiferromagnet models will be explained.
Based on a joint work with Alan Sokal and Jan Swart.
Schreiber Building Room 309 at 14:30.

Monday, December 2

Manfred Einsiedler, ETH Zurich
Equidistribution and p-adic dynamics
Abstract:
We will discuss the joint equidistribution of the direction of integer points on large spheres and the shape of the lattice in the orthogonal complement. As we will see this problem can be analyzed using p-adic (or Hecke-) dynamics and theorems of Mozes and Shah resp. Lindenstrauss and myself. We will define all relevant spaces and terms. This is joint work with Aka and Shapira.
Schreiber Building Room 309 at 14:30.

Monday, December 9

Jesse Goodman, Technion
The gaps left by a Brownian motion
Abstract:
Run a Brownian motion in a compact domain for a long time. What are the shapes of the large gaps remaining? This question, while simple to state, has surprising connections to harmonic analysis, potential theory, and complex geometry.
The answer depends strongly on the dimension, and in three or more dimensions the shapes of gaps can be characterised in a largely deterministic way. I will also discuss the conjectured picture in two dimensions, where long range correlations bring the problem into the realm of multiscale, hierarchical models.
Schreiber Building Room 309 at 14:30.

Monday, December 16

Dima Ioffe, Technion
Low temperature interfaces, layering transitions and Ferrari-Spohn diffusions
Abstract:
2+1 Solid-on-solid interfaces are designed to mimic phase boundaries in genuine three dimensional models of statistical mechanics (e.g. Ising), and to facilitate analysis of various interfacial phenomena, such as fluctuations, entropic repulsion, facet formation and wetting.
For a class of flat SOS type interfaces large scale statistics of level sets, in particular the number of macroscopic size layers, could be quantified in terms of optimal shapes for limiting variational problems. Fluctuations of random level lines around limiting shapes are expected to obey diffusive scaling in the bulk and 1/3-scaling near the edges, which should be reminiscent of 1/3-scaling derived by Ferrari ans Spohn for constrained Brownian motion above circular barriers. Invariance principle to Ferrari-Spohn diffusions is derived for a class of random walks above hard walls with area-type tilts.
Based on joint works with S.Shlosman, F.Toninelli and Y.Velenik.
Schreiber Building Room 309 at 14:30.

Monday, December 23

Ariel Yadin, Ben Gurion University
Towards an inverse Kleiner Theorem for finitely generated groups
Abstract:
We study Lipschitz harmonic functions on finitely generated groups. (These are naturally probabilistic objects due to the fact that harmonic functions evaluated on a random walk are martingales.) One of the central places where such functions arise is in the "new" proof of Gromov's theorem regarding polynomial growth groups by Kleiner, later simplified by Shalom and Tao. Gromov's theorem asserts that any finitely generated group of polynomial growth is almost nilpotent, thus transforming a seemingly metric property that depends on the choice of generators to an algebraic property. In 2007 Kleiner gave a new proof of this theorem. The main part is to prove that for any finitely generated group of polynomial growth the space of Lipschitz harmonic functions on the group is of finite dimension.
Our point of departure is the study of the possibility of an "inverse" Kleiner's theorem: namely, is it true that if a finitely generated group has a finite dimensional space of Lipschitz harmonic functions, then that group has polynomial growth. This being work in progress, we have not been able to settle this question, but we have made progress towards such a theorem. The main result I will speak about is:
Theorem: If the space of Lipschitz harmonic functions on a finitely generated group G is finite dimensional, then there exists a finite index subgroup H of G such that all Lipschitz harmonic functions restricted to H are homomorphisms of H into the additive complex number group plus a constant (characters + constants).
As a corollary, we obtain that the dimension of the space of Lipschitz harmonic functions on a group is either infinite or some number d that is a group invariant. I will also explain why these are steps towards proving an inverse Kleiner's theorem.
This is joint work with Tom Meyerovitch.
Schreiber Building Room 309 at 14:30.

Monday, December 30

Boris Solomyak, University of Washington
On the Fourier asymptotics of self-similar measures
Abstract:
We consider the asymptotics of the Fourier Transform (FT) of Bernoulli convolutions and other self-similar measures. It is known since the work of Erdos and Salem in the 1930-40's that the FT of the Bernoulli convolution with contraction ratio r tends to zero at infinity if and only if 1/r is not a Pisot-Vijayaraghavan (PV) number. However, no quantitative estimates for the decay of the FT follow from the proof. The work of Erdos and Kahane on the power decay for "most" r has recently found new applications, e.g. in the work of P. Shmerkin, who proved absolute continuity of Bernoulli convolutions for all but zero-dimensional set of parameters r in (0.5.1). I will start with the history, and then discuss some generalizations (joint work with Shmerkin) and applications to the spectral theory of substitution dynamical systems (joint work with A.Bufetov).
Schreiber Building Room 309 at 14:30.

Double seminar! Note special time! Each talk will be 50 minutes.
Monday, January 6

Rodrigo Treviño, Tel Aviv University
Keith Merrill, Brandeis University
Treviño title: Controlling deforming geometries
Treviño Abstract:
Masur’s criterion for unique ergodicity for translation flows on flat surfaces says that if the Teichmuller orbit of a flat surface is recurrent to a compact set of the moduli space, then the translation flow defined by that surface is uniquely ergodic. The recurrence to a compact set in this criterion amounts to the geometry of the surface not degenerating as one applies the Teichmuller deformation. I will present a theorem which applies to any flat surface of finite area which says that if the geometry of a flat surface undergoing Teichmuller deformation can be more or less controlled, then the translation flow is uniquely ergodic. In the case of the surface being compact, this implies Masur’s criterion and a bit more. The proof is inspired by Forni’s proof for the spectral gap of the Kontsevich–Zorich cocycle. Time permitting, I will discuss the proof (which is in my opinion the more magical part of this) and discuss some possible future directions of this approach.
Merrill title: Diophantine Approximation and Dynamics
Merrill Abstract:
In this talk I will discuss the central questions of the field of Diophantine approximation, which seeks to quantify the density of subsets of a metric space. We will focus on the classical case of the density of rational numbers in the real numbers. Then we will discuss and answer these same questions in the context of rational points on spheres and, time permitting, rational points on general quadratic hyper surfaces. The proof techniques involve an analysis of orbits of a certain flow on certain homogeneous spaces. No background knowledge will be assumed.
This is joint work with Lior Fishman, Dmitry Kleinbock, and David Simmons.
Schreiber Building Room 309 at 14:10.

Monday, January 13

Pankaj Vishe, University of York
Inhomogeneous multiplicative Littlewood conjecture and logarithmic savings
Abstract:
We use the dynamics on SL(3,R)/SL(3,Z) to get logarithmic savings in the inhomogeneous multiplicative Littlewood setting. This is a joint work with Alex Gorodnik.
Schreiber Building Room 309 at 14:30.





Spring Semester 2013


Monday, February 25
Special pre-semester seminar!

Hitoshi Nakada, Keio University, Japan
On cost functions for some Euclidean type algorithms over F_q[X]^d
Abstract:
We consider some cost functions of Euclidean type algorithms. We show that the law of large numbers hold for the bit complexity and the fine bit complexity of accelerated fully subtractive over F_q -coefficients d polynomials. For this purpose, we consider the associated dynamical systems on F_q ((X^{-1})) ^d.
Schreiber Building Room 309 at 14:30.

Monday, March 4

Bryna Kra, Northwestern University
Rectangular tiling factors of R^d actions
Abstract:
We study the space of tilings of R^d for a collection of d-dimensional rectangles. Rudolph showed that there exist 2^d such tiles such that any free measure preserving R^d action has a tiling from this space as a factor map, and we show that d+1 tiles suffices. Furthermore, by studying the geometric properties of the tilings in R^2, we show that this result is sharp in two dimensions. This is joint work with A. Quas and A. Sahin.
Schreiber Building Room 309 at 14:30.

Monday, March 11

Cyrille Lucas, Weizmann Institute
Uniform Internal Diffusion Limited Aggregation
Abstract:
Internal Diffusion Limited Aggregation, or iDLA, is a growth model in which random sets are constructed recursively. At each step, a random walk starts at the origin and the first point it visits outside the cluster is added to the aggregate. In our modified version of this model, the new random walk starts in a random point distributed uniformly on the aggregate. We will present the major differences between the two models and show the convergence of the normalised Uniform iDLA cluster towards the Euclidean unit ball.
Joint work with Itai Benjamini, Hugo Duminil-Copin, Gady Kozma.
Schreiber Building Room 309 at 14:30.

Monday, March 18

Dan Romik, University of California, Davis
Loop percolation, pipe percolation and random noncrossing matchings
Abstract:
The talk will be about the same family of random noncrossing matchings introduced in my colloquium talk earlier on the same day, but the discussion will be self-contained and no previous knowledge will be assumed. My goal will be to show that these random noncrossing matchings appear as the connectivity patterns associated with a second (and seemingly unrelated) type of percolation called "pipe percolation". Pipe percolation was previously defined only on a cylindrical geometry (or equivalently as a discrete-time random walk on the finite set of generators of the Temperley-Lieb algebra), but extending the definition to the setting of the entire plane brings up new and subtle issues. I will define the process rigorously as a continuous-time Markov process taking values in the space of noncrossing matchings of the integers. The proof that the construction works will use ideas from ergodic theory and the theory of interacting particle systems. I will also discuss bounds on the matching distance, which relate to the well-known open problem of the "5/48" percolation critical exponent.
Schreiber Building Room 309 at 14:30.

Monday, March 25

Passover break - No seminar.

Monday, April 1

Passover break - No seminar.

Monday, April 8

Ronen Eldan, Weizmann Institute
On the connection between the spectral gap of convex bodies and the variance conjecture.
Abstract:
We consider the uniform measure over a high-dimensional isotropic convex body. We prove that, up to logarithmic factors, the isoperimetric minimizers are ellipsoids. Equivalently, we show that up to a logarithmic factor, the "worst-behaving" functions in the corresponding poincare inequality are quadratic functions. We thus establish a connection between two well-known conjectures regarding the uniform measure over a high dimensional convex body, namely the Thin-Shell conjecture and the conjecture by Kannan-Lovasz-Simonovits (KLS) , showing that a positive answer to the former will imply a positive answer to the latter (up to a logarithmic factor). Our proof relies on the analysis of the eigenvalues of a certain random-matrix-valued stochastic process related to a convex body.
Schreiber Building Room 309 at 14:30.

Monday, April 15

Memorial day - No seminar.

Monday, April 22

Omer Tamuz, Weizmann Institute
The Furstenberg Entropy Realization Problem
Abstract:
Random walks on groups and harmonic functions on groups are intimately related to a generalization of measure preserving group actions called "stationary group actions". An important invariant of stationary group actions is their Furstenberg Entropy. The Furstenberg Entropy realization problem is the question of determining the range of possible entropy values realizable for a given random walk.
The talk will include an introduction to this field, an overview of what (little) is known, and some new results. Joint work with Yair Hartman.
Schreiber Building Room 309 at 14:30.

Monday, April 29

Gidi Amir, Bar-Ilan University
Multiple excited random walk, excited Mob and leftover environments
Abstract:
We discuss the model of multiple excited random walk on Z, which is a model of self-interacting random walk on Z which generalizes the notion of random walk in random environment (in this model the walk changes the environment as it walks through it). We will survey results on this model (and higher dimensional analogues) regarding transience, recurrence and positive speed of such walks, and discuss some of the techniques involved. In particular we will discuss some related branching processes and explain how some new observations on these processes can lead to a 0-1 law for directional transience of such walks.
We will then discuss new ideas, that allow us to conclude exact criterions for recurrence, transience and positive speed of such walks on the "leftover" environments - the environments left over after a transient walker goes to infinity. For our analysis we introduce the idea of multiple walkers walking in the same environment, and show how their movement can be analyzed and used to understand the leftover environments.
No prior knowledge on excited random walks or random walk in random environment is assumed.
This is based on joint works with Tal Orenshtein and with Tal Orenshtein and Noam Berger.
Schreiber Building Room 309 at 14:30.

Monday, May 6

Yuri Lima, Weizmann Institute
A Marstrand theorem for subsets of integers
Abstract
Schreiber Building Room 309 at 14:30.

Monday, May 13

Asaf Nachmias, University of British Columbia
Random walks on planar graphs via circle packings
Abstract:
I will describe two results concerning random walks on planar graphs and the connections with Koebe's circle packing theorem (which I will not assume any knowledge of):
1. A bounded degree planar triangulation is recurrent if an only if the set of accumulation points of its circle packing is a polar set (that is, has zero logarithmic capacity). This extends a result of He and Schramm who proved recurrence (transience) when the set of accumulation points is empty (a closed Jordan curve). Joint work with Ori Gurel-Gurevich and Juan Souto.
2. The Poisson boundary (the space of bounded harmonic functions) of a transient bounded degree triangulation of the plane is characterized by the topological boundary obtained by circle packing the graph in the unit disk. In other words, any bounded harmonic function on the graph is the harmonic extension of some measurable function on the boundary of the unit disc. Joint work with Omer Angel, Martin Barlow and Ori Gurel-Gurevich.
Schreiber Building Room 309 at 14:30.

Monday, May 20

Omer Angel, University of British Columbia
Evolving sets and hitting times
Abstract:
I will review the method of evolving sets, describe a new connection to hitting times, and show that in certain cases, permutations can only accelerate hitting.
Schreiber Building Room 309 at 14:30.

Monday, May 27

Pascal Maillard, Weizmann Institute
Performance of the Metropolis algorithm on a disordered tree
Abstract:
I will present recent results obtained with Ofer Zeitouni on the performance of the Metropolis algorithm on a branching random walk tree. Based on our recent article available at arXiv:1304.0552.
Schreiber Building Room 309 at 14:30.

Monday, June 3

John Smillie, Cornell University
Do analogs of Ratner's theorems hold for strata of translation surfaces?
Abstract:
Questions about polygonal billiards lead to the study of moduli spaces of translation surfaces or "strata". There are intriguing parallels between strata and homogeneous spaces and there has been a fruitful migration of ideas from homogeneous dynamics to "Teichmuller dynamics" on strata. One pivotal question is the extent to which Ratner theory applies to strata. I will discuss positive results of Eskin and Mirzakhani and describe negative results obtained with Barak Weiss.
Schreiber Building Room 309 at 14:30.

Monday, June 10

Amir Dembo, Stanford University
Persistence Probabilities
Abstract:
Persistence probabilities concern how likely it is that a stochastic process has a long excursion above a fixed level and what are the relevant scenarios for this behavior. Power law decay is expected in many cases of physical significance and the issue is to determine its power exponent parameter. I will survey recent progress in this direction (jointly with Jian Ding, Fuchang Gao, and Sumit Mukherjee), dealing with random algebraic polynomials of independent coefficients, iterated partial sums and other auto-regressive sequences, and with the solution to heat equation initiated by white noise.
Schreiber Building Room 309 at 14:30.

Monday, June 17

Elchanan Mossel, University of California, Berkeley
Robust Optimality of Gaussian Noise Stability
Abstract:
In 1985 C. Borell proved that under the Gaussian measure, half-spaces are the most stable sets. I will present two new proofs of this result. The first proof solves a long standing open problem by showing that half-spaces are the unique optimizers. It also provides quantitative dimension independent versions of uniqueness, showing that a set which is almost optimally noise stable must be close to a half-space. This has a number of application in theoretical computer science and social choice. The first proof also allows to answer a long standing open problem by Ledoux. The second proof is proved by induction on dimension in the discrete cube which allows to derive a proof of "Majority is Stablest" in the "Sum of Squares Proof System" thus aswering a recent question regarding semi-definite relaxations. Based on joint works with J. Neeman and Anindya De.
Schreiber Building Room 309 at 14:30.

Monday, June 24

Pascal Vanier, Hebrew University
Characterizations of periods of multidimensional SFTs
Abstract:
In this talk, we will introduce several notions of periodicity for higher dimensional shifts and give computational characterizations of sets of periods of SFTs for some of them. We will in particular focus on points with a finite orbit, the straightforward generalization of periodicity from dimension 1. We will show that sets of periods of multidimensional SFTs correspond exactly to the languages of the computational complexity class NE (languages recognizable in nondeterministic linear exponential time). In order to attain this result we will first focus on the intermediate notion of horizontal periodicity, whose characterization is easier to achieve.
Schreiber Building Room 309 at 14:30.





Fall Semester 2012


Monday, October 15
Special pre-semester seminar!

Ram Band, University of Bristol
Spectral Geometry on Graphs
Abstract:
The talk's theme is the extraction of geometric information about graphs (metric or combinatorial) from the spectra of the graph's Schroedinger operators (continuous or discrete), and from the distribution of sign changes on the corresponding eigenfunctions. These include questions such as e.g., the ability to "hear the shape of the graph"; the extent to which the spectral sequence and the sequence of the number of sign changes (or number of nodal domains) complement or overlap each other; the derivation of topological information from the study of the response of the spectrum to variation of scalar or magnetic potentials on the graph, etc.
In the present talk I shall illustrate this research effort by reviewing several results I obtained recently. The first example answers the question "Can one count a tree?" which appears in the following context: It is known that the number of sign changes of the eigenfunction on tree graphs equals to the position of the corresponding eigenvalue in the spectrum minus one. Is the reverse true? If yes, one can tell a tree just by counting the number of its sign changes. For the proof I shall introduce an auxiliary magnetic field and use a very recent result of Berkolaiko and Colin de Verdiere to connect the spectrum and the number of sign changes. Next, I will discuss the band spectrum obtained by varying the magnetic phases on the graph. I will prove that the magnetic band-to-gap ratio (quality of conductance) is a universal topological quantity of a graph. This result highlights the spectral geometric importance of this invariant and sheds a new light on previous works about periodic potentials on graphs.
The talk contains content of a work in progress with Gregory Berkolaiko. All concepts will be explained and no previous knowledge of the topic is required.
Schreiber Building Room 309 at 14:30.

Monday, October 22

Yinon Spinka, Tel Aviv University
The Random Graph-Homomorphism Model
Abstract:
We discuss the random graph-homomorphism model introduced by Benjamini, Häggström and Mossel (2000). This model generalizes the simple random walk on Z, and therefore, is sometimes referred to as a G-indexed random walk or a random height function on a graph. We begin by introducing the model (no prior knowledge is assumed) and by giving some examples. We then give a short survey of known results and discuss some conjectures. The main questions of interest are the variance at a given vertex and the range a of random homomorphism. We then continue on to our results. We study two certain families of graphs (which can be seen as modifications of a torus or a line segment) and show a sharp transition in the behavior of a typical homomorphism. The main results will be stated and outlines and ideas of some proofs will be given. This work answers a question posed by Benjamini, Yadin and Yehudayoff (2007).
Schreiber Building Room 309 at 14:30.

Monday, October 29

Michael Krivelevich, Tel Aviv University
Random subgraphs of large minimum degree graphs
Abstract:
Consider the following very general model of random graphs: let G be a finite graph of minimum degree at least k, for k tending to infinity, and form a random subgraph G_p of G by taking each edge of G with probability p=p(k), independently. What can be said about typical properties of such random graph? This model covers a lot of ground, including binomial random graphs G(k+1,p), random subgraphs of the k-dimensional binary cube Q^k, random subgraphs of k-regular expanders etc. Generality has its price, and some clasical questions from the theory of random graphs (appearance of a fixed subgraph, chromatic number etc.) become irrelevant, while some others are probably just too hard. Still, there is quite a number of attractive problems that appear to be approachable. In this talk, I will report about our recent results on some of them.
Based on joint works with (subsets of) Alan Frieze, Choongbum Lee, Benny Sudakov.
Longer seminar! Note special time!
Schreiber Building Room 309 from 14:30 to 16:00!

Monday, November 5

Itai Benjamini, Weizmann Institute
Euclidean vs graph metric
Abstract:
We will discuss how well large (random and not random) graphs approximate other spaces such as the Euclidean plane.
Schreiber Building Room 309 at 14:30.

Monday, November 12

Yogeshwaran Dhandapani, Technion
On the topology of some random complexes built over stationary point processes
Abstract:
There has been recent interest in understanding the homology of random simplicial complexes built over point processes, primarily motivated by problems in applied algebraic topology. I shall describe our new results about the growth of homology groups of Cech and Vietoris-Rips complexes built over general stationary point processes. Both these complexes have points of the point process as vertices and the faces are determined by some deterministic geometric rule. The aim of the talk shall be to explain the quantitative differences in the growth of homology groups measured via Betti numbers between the Poisson point process and other point processes which exhibit repulsion such as the Ginibre ensemble, zeros of Gaussian analytic functions, perturbed lattice etc. I shall also try to hint at the proof techniques which involve detailed analysis of subgraph and component counts of the associated random geometric graphs and are applicable to similar functionals of point processes such as Morse critical points. This is a joint work with Prof. Robert Adler.
Schreiber Building Room 309 at 14:30.

Monday, November 19

Benjamin Weiss, The Hebrew University of Jerusalem
On "a curious conjugacy invariant" of Halmos
Abstract:
One of the problems that Halmos posed at the end of his classic lectures on ergodic theory concerns a curious conjugacy invariant that he suggested might distinguish between ergodic toral automorphisms. I will describe this invariant and report on some recent results of A. Quas and T. Soo on the ergodic universality of toral automorphisms which relate to this invariant. In addition I will indicate an alternative argument which will suffice for the application to the Halmos problem.
Schreiber Building Room 309 at 14:30.

Monday, November 26

Ron Rosenthal, The Hebrew University of Jerusalem
Random walks, spectrum and homology of complexes
Abstract:
There are well known connections between the random walk on a graph, and its topological and spectral properties. Here we define a stochastic process on higher dimensional simplicial complexes, which reflects their homological and spectral properties in a parallel way. This leads to high dimensional analogues (not all of which hold!) of classical theorems of Kesten, Alon-Boppana, and others. No previous knowledge is assumed. Joint work with Ori Parzanchevski.
Schreiber Building Room 309 at 14:30.

Monday, December 3

Action Now meeting at Bar-Ilan University - No seminar.

Monday, December 10

Brad Rodgers, University of California, Los Angeles
Statistics of the zeros of the zeta function: mesoscopic and macroscopic phenomena
Abstract:
We review the well known microscopic correspondence between random zeros of the Riemann zeta-function and the eigenvalues of a random unitary or Hermitian matrix, and discuss evidence that this correspondence extends to larger mesoscopic collections of zeros or eigenvalues. In addition, we discuss interesting phenomena that appears in the statistics of even larger macroscopic collections of zeros. The terms microscopic, mesoscopic, and macroscopic are from random matrix theory and will be defined in the talk. If time permits we will raise some questions about general classes of point processes, which model the distribution of zeros.
This talk is based in part on results in the papers arXiv: 1203.3275 math.NT, and arXiv: 1205.0303 math.PR.
Schreiber Building Room 309 at 14:30.

Monday, December 17

Oren Louidor, University of California, Los Angeles
Isoperimetry in Supercritical Percolation.
Abstract:
We consider the unique infinite connected component of supercritical bond percolation on the square lattice and study the geometric properties of isoperimetric sets, i.e., sets with minimal boundary for a given volume. For almost every realization of the infinite connected component we prove that, as the volume of the isoperimetric set tends to infinity, its asymptotic shape can be characterized by an isoperimetric problem in the plane with respect to a particular norm. As an application we then show that the anchored isoperimetric profile with respect to a given point as well as the Cheeger constant of the giant component in finite boxes scale to deterministic quantities. This settles a conjecture of Itai Benjamini for the plane. Joint work with M. Biskup, E. Procaccia and R. Rosenthal.
Schreiber Building Room 309 at 14:30.

Monday, December 24

Yuval Peres, Microsoft Research
Balanced self-interacting random walks.
Abstract:
It is well known that a random walk in d>2 dimensions where the steps are i.i.d. mean zero and fully supported (not restricted to a hyperplane), is transient. In a recent elegant paper, Benjamini, Kozma and Schapira (2011) asked if we still must have transience when each step is chosen from either \mu_1 or \mu_2 based on the past, where \mu_1 and \mu_2 are fully supported mean zero distributions in dimension d>2. (e.g. we could use \mu_1 if the current state has been visited before, and \mu_2 otherwise), We answer their question, and show the answer can change when we have three measures instead of two. To prove this, we will adapt the classical techniques of Lyapunov functions and excessive measures to this setting. No prior familiarity with these methods will be assumed, and they will be explained in the talk. Many open problems remain in this area, even in two dimensions.
Lecture based on joint work with Serguei Popov (Campinas, Brazil) and Perla Sousi (Cambridge, UK).
Schreiber Building Room 309 at 14:30.

Monday, December 31

Fedor Nazarov, Kent State University
Littlewood-Offord-Turan estimate for the number of real zeroes of a random polynomial with i.i.d. coefficients
Abstract:
We show that the average number of real zeroes of any random polynomial of degree $n$ with independent identically distributed coefficients does not exceed $C\log^4 n$ with some absolute $C>0$. The proof follows closely the paper by Littlewood and Offord published in 1942, where the case of $\pm 1$ coefficients was considered. Our main deviation from their scheme is using the Turan lemma from Turan's 1953 book in place of the pointwise "anticoncentration" estimate used by Littlewood and Offord. The result is on par with the original Littlewood-Offord bound $C\log^2 n$ but still short of the would be optimal estimate $C\log n$, which is widely believed to hold and whose validity is known for all sufficiently regular distributions. This is a joint work with Mikhail Sodin.
Longer seminar! Note special time!
Schreiber Building Room 309 from 14:30 to 16:00!

Monday, January 7

Tom Meyerovitch, Ben-Gurion University
On equivariant thinning, allocation and matching schemes for Poisson processes
Abstract:
Poisson thinning, allocation and matching are ``natural'' operations on a realization of a Poisson process which have been considered in the literature. A Poisson thinning, for instance, is a rule for selecting a subset of the points in the Poisson process which are equal in distribution to a lower intensity Poisson process. There are interesting and non-trivial constructions of isometry-equivariant Poisson thinning, Poisson allocation and Poisson matching.
In this talk I will consider the existence of operations of the above types which are equivariant with respect to a group of measure-preserving transformations other than isometries. Evans proved that the only linear transformations which admit equivariant Poisson thinning are isometries. I will show that no equivariant thinning, allocation or matching is possible with respect to any conservative and ergodic measure preserving transformation. My proof is based on an ergodicity result which uses Kean's classic ``ergodic multiplier theorem''. The definitions involved will be given during the talk, no background with Poisson processes is assumed.
Schreiber Building Room 309 at 14:30.

Monday, January 14

Jay Rosen, The City University of New York
Markovian loop soups, permanental processes and isomorphism theorems
Abstract:
We show how to construct loop soups for general Markov processes and explain how loop soups offer a deep understanding of Dynkin's isomorphism theorem, and beyond.
Schreiber Building Room 309 at 14:30.

Monday, January 21

Piotr Miłoś, University of Bath and University of Warsaw
Heavy-tailed branching Lévy motion in inhomogeneous potential
Abstract:
In my talk I will present results concerning a branching system as follows. Particles evolve according to a real-valued Lévy motion with heavy tails and branch according to a dyadic branching law. We consider three different regimes of the branching rate:
- constant in space, then the system size grows exponentially.
- \beta \log(x), then the system size grows as exp(exp(\beta/(e \alpha)t)).
- \log^{1+\epsilon}(x), then almost surely the system explodes, i.e. produces an infinite number of particles in finite time.
These are joint results with Simon Harris (University of Bath)
Schreiber Building Room 309 at 14:30.





Special Summer Seminars 2012


Monday, July 2

Oren Louidor, University of California, Los Angeles
Complete Survival for the Biased Voter Model on Regular Trees
Abstract:
We consider the Biased Voter Model (also known as the Williams Bjerknes model) on the d-regular tree and prove that local survival implies asymptotic consensus (or complete survival). Complete survival and fixation are also considered in the case of general vertex transitive graphs, where the initial configuration is chosen according to a distribution which is automorphism invariant and/or ergodic. Joint work with Ran Tessler and Alexander Vandenberg-Rodes.
Schreiber Building Room 309 at 14:30.

Monday, July 9

Brendan Farrell, California Institute of Technology
The Jacobi ensemble and discrete uncertainty principles
Abstract:
The Jacobi ensemble is the third classical ensemble of random matrix theory: it describes the eigenvalue distribution of a form of Gaussian random matrix known as a MANOVA matrix. Matrices of this form are related to angles between random subspaces and, in particular, to discrete uncertainty principles. We present initial universality results for MANOVA matrices and for random submatrices of the discrete Fourier transform matrix. This is partially joint work with László Erdős.
Schreiber Building Room 008 at 14:30.

Monday, August 20

Todd Kemp, University of California, San Diego
Liberation of Random Projections
Abstract
Note special time!
Schreiber Building Room 309 at 13:00.





Spring Semester 2012


Monday, March 5

Barak Weiss, Ben-Gurion University
The Mordell-Gruber spectrum and homogeneous dynamics.
Abstract:
Given a lattice in euclidean space, its Mordell constant is the supremum of the volumes (suitably normalized) of central symmetric boxes with sides parallel to the axes, containing no nonzero lattice points. This is a well-studied quantity in the geometry of numbers, and is invariant under the action of diagonal matrices on the space of lattices. In joint work with Uri Shapira, using dynamics of the diagonal group we obtain new results about the set of possible values of the Mordell constant (the so-called Mordell-Gruber spectrum).
Schreiber Building Room 309 at 14:30.

Monday, March 12

Noam Berger, The Hebrew University of Jerusalem
New conditions for ballisticity for RWRE
Abstract:
In 2000, Sznitman proved that a uniformly elliptic RWRE is ballistic if a certain back-stepping probability decays exponentially. Later he weakened this condition to stretched exponential decay. In the talk I will show that in dimensions four and higher, the condition can be relaxed to (high degree) polynomial decay. All terminology will be explained in the talk. Based on joint work with Drewitz (Zurich) and Ramirez (Santiago de Chile).
Schreiber Building Room 309 at 14:30.

Monday, March 19

Agelos Georgakopoulos, Technical University, Graz
Random walks, electrical networks, and the role of the Poisson boundary
Abstract:
I will present some basic facts concerning electrical networks and their connections to random walk. Then, I will present a new result relating Dirichlet harmonic functions on an infinite graph to its Poisson boundary. The talk will be accessible to the non-expert.
Joint work with V. Kaimanovich.
Schreiber Building Room 309 at 14:30.

Monday, March 26

Jon Aaronson, Tel Aviv University
On the categories of weak mixing in infinite measure spaces
Abstract:
I will discuss some propositions such as "in general" an infinite measure preserving trasformation is subsequence rationally weakly mixing, but not rationally weakly mixing.
Schreiber Building Room 309 at 14:30.

Monday, April 2

Zemer Kosloff, Tel Aviv University
Power Weakly Mixing Transformations
Abstract
Schreiber Building Room 309 at 14:30.

Monday, April 9

Passover break - No seminar.

Monday, April 16

Mikhail Sodin, Tel Aviv University
Random nodal portraits
Abstract:
We describe the progress in understanding the zero sets of smooth Gaussian random functions of several real variables. The primary examples are various ensembles of Gaussian real-valued polynomials (algebraic or trigonometric) of large degree, and smooth Gaussian functions on the Euclidean space with translation-invariant distribution. The fundamental question is the one on the asymptotic behaviour of the number of connected components of the zero set. This can be viewed as a statistical version of Hilbert's 16th problem.
We start with an intriguing heuristics suggested by Bogomolny and Schmit, which relates nodal portraits of 2D Gaussian monochromatic waves to bond percolation on the square lattice. Then we explain how Ergodic Theorem and rudimentary harmonic analysis help to find the order of growth of the typical number of connected components of the zero set. We will mention a number of basic questions, which remain widely open.
The talk is based on joint works Fedor Nazarov.
Longer seminar! Note special time!
Schreiber Building Room 309 from 14:30 to 16:00!

Monday, April 23

Ross Pinsky, Technion
Probabilistic and Combinatorial Aspects of the Card-Cyclic to Random Insertion Shuffle
Abstract
Schreiber Building Room 309 at 14:30.

Monday, April 30

Sasha Sodin, IAS, Princeton
A 2D gradient model with non-convex interaction
Abstract:
The talk will be about a statistical mechanics model on the 2D lattice. We shall define it, and discuss the connection to height models, the six-vertex model, and dipole gas (all of these will be defined as well). Also, we shall discuss the conjectured phase diagram, and explain which parts are rigorously justified.
Based on discussions with David Brydges and Tom Spencer.
Schreiber Building Room 309 at 14:30.

Monday, May 7

Yan V Fyodorov (Queen Mary, University of London)
Freezing Transition: from 1/f landscapes to Characteristic Polynomials of Random Matrices and the Riemann zeta-function
Abstract:
In the talk (based on a joint work with G Hiary and J Keating; arXiv:1202.4713) I will argue that the freezing transition scenario, previously conjectured to take place in the statistical mechanics of 1/f-noise random energy models, governs, after reinterpretation, the value distribution of the maximum of the modulus of the characteristic polynomials of large random unitary (CUE) matrices. I then conjecture that the results extend to the large values taken by the Riemann zeta-function over stretches of the critical line s=1/2+it of constant length, and present the results of numerical computations of the large values of $\zeta(1/2+it)$. The main purpose is to draw attention to possible connections between the statistical mechanics of random energy landscapes, random matrix theory, and the theory of the Riemann zeta function.
Schreiber Building Room 309 at 14:30.

Monday, May 14

Mark Rudelson, University of Michigan
Row products of random matrices
Abstract:
We study spectral and geometric properties of a certain class of random matrices with dependent rows, which are constructed from random matrices with independent entries. For K matrices of size d times n we define the row product as a matrix of the size d^K times n, whose rows are entry-wise products of the rows of the original matrices. Such constructions arise in several computer science problems. Simulations show that, despite the dependency between the entries, these matrices behave like random matrices of the same size with independent entries. We will discuss how far this similarity can be extended.
Schreiber Building Room 309 at 14:30.

Double seminar! Note special time! Each talk will be 50 minutes.
Monday, May 21

Elchanan Mossel, University of California, Berkeley
Mira Shamis, Institute for Advanced Study, Princeton
Mossel title: On reverse hypercontractive inequalities.
Mossel Abstract:
A hyper-contractive inequality for an operator T states that |Tf|_q ≤ |f|_p where q > p > 1 for all functions f. Hyper contractive inequalities play a crucial role n discrete Fourier analysis and in probabilistic applications. A reverse hyper-contractive inequality for the operator T states that |Tf|_q ≥ |f|_p for q < p < 1 (q and p can be negative) and all strictly positive functions f. The first reverse hyper-contractive inequalities were proved by Borell more than 2 decades ago. While these inequalities may look obscure, they have been used for the solution of a number of problems in the last decade. I will survey applications of the inequalities and discuss new results relating reverse hyper-contractive inequalities to hyper-contractive, Log-Sobolev and Poincare inequalities as well as some new applications. This is a joint work with K Oleszkiewicz (Warsaw) and A Sen (Cambridge).
Shamis title: Anderson localization for non-monotone Schroedinger operators.
Shamis Abstract:
We show how the fractional moment method of Aizenman and Molchanov can be applied to a class of Anderson-type models with non-monotone potentials, to prove (spectral and dynamical) localization. The main new feature of our argument is that it does not assume any a priori Wegner-type estimate: the (nearly optimal) regularity of the density of states is established as a byproduct of the proof. The argument is applicable to finite-range alloy-type models and a class of operators with matrix-valued potentials.
(joint work with A. Elgart and S. Sodin)
Schreiber Building Room 309 at 14:10.

Monday, May 28

Israel Mathematical Union meeting - No seminar.

Monday, June 4

Noga Alon, Tel Aviv University
The chromatic number of random Cayley graphs
Abstract:
The study of random Cayley graphs of finite groups is related to the investigation of Expanders and to problems in combinatorial Number Theory and in Information Theory. I will discuss this topic, focusing on the question of estimating the chromatic number of a random Cayley graph of a given group with a prescribed number of generators. One representative result is the fact that if p is a large prime and S is a random subset of at most 0.5 log_2 p members of Z_p, then the chromatic number of the Cayley graph of Z_p with respect to S is almost surely exactly 3.
Schreiber Building Room 309 at 14:30.

Double seminar! Note special time and place! Each talk will be 50 minutes.
Monday, June 11

Loren Coquille, Université de Genève
Vincent Beffara, ENS Lyon
Coquille title: On the Gibbs measures of the 2d Ising and Potts models.
Coquille Abstract:
In the late 1970s, in two celebrated papers, Aizenman and Higuchi independently established that all infinite-volume Gibbs measures of the 2d Ising model are a convex combination of the two pure phases. After introducing the relevant definitions and concepts needed to understand the physical content of this result, I will present a finite-volume, quantitative analogue (implying the classical claim). The scheme of the proof seems more natural and provides a better picture of the underlying physical phenomenon. (Joint work with Yvan Velenik). Moreover, this new approach is substantially more robust : I will explain in parallel how it extends to the Potts model, and what are the major technical and physical difficulties to overcome. (Joint work with Hugo Duminil-Copin, Dima Ioffe and Yvan Velenik).
Beffara title: Self-interacting random walks and growth processes.
Beffara Abstract:
We will present a surprising link between the once-reinforced random walk, which is a self-interacting process on a lattice, and a growth system related to first-passage percolation on the same lattice. This gives a very precise description of the scaling limit of the walk, which exhibits a deterministic asymptotic shape.
Schreiber Building Room 006 at 14:10.

Double seminar! Note special time! Each talk will be 50 minutes.
Monday, June 18

Nayantara Bhatnagar, University of California, Berkeley
Jonathan Hermon, Weizmann Institute
Bhatnagar title: Reconstruction on Trees.
Bhatnagar Abstract:
For spin systems on a tree, roughly, the reconstruction problem is to determine whether correlations persist between vertices deep inside the tree and the root. Reconstruction on trees plays an important role in explaining threshold phenomena in random constraint satisfaction problems on sparse random graphs.
In the talk, I will speak about results on the threshold for reconstruction for colorings and algorithms for finding thresholds (work that is joint with Vera, Vigoda, and Weitz and with Maneva). I'll also mention results on the reconstruction threshold for independent sets (work that is joint with Sly and Tetali).
Hermon title: Social Connectivity of Random Mobile Networks.
Hermon Abstract:
Joint work with Gady Kozma and Itai Benjamini - In this work we study questions related to the evolution of random mobile social networks in various graphs. In particular we study the time it takes them to become connected and for a giant component to emerge. We do so by considering a model in which there are many random walkers (which are the people of the social network) performing independent random walks on a given graph, where new acquaintances occur when walkers collide. The main reasults are polylogarithmic bounds on the social connectivity time when the underlying graph on which the walkers walk is of bounded degree. Other results concerning the complete graph (and relations to G(n,p)), expanders, the cycle and Z^{d} (and relations to percolation) might be discussed as time permits.
Schreiber Building Room 309 at 14:10.





Fall Semester 2011


Monday, October 31

Daniel Johannsen, Tel Aviv University
The Degree Sequence of Random Planar Maps
Abstract:
In this talk we study how typical specimen of various classes of planar maps (i.e., embedded planar graphs) look like. In particular, we are interested in the degree sequence of a random map drawn from all maps of equal size. For ordinary random maps, it is known that the expected number of vertices of a fixed degree is linear in the number of edges of that map. Moreover, this number is sharply concentrated around its expectation for which an asymptotic formula (depending on the given degree) exists. We see how this result can be transferred to other classes of random maps like those that are biconnected, 3-connected, loopless, bridgeless, or simple.
This is joint work with Konstantinos Panagiotou.
Schreiber Building Room 309 at 14:30.

Monday, November 7

Ohad Feldheim, Tel Aviv University
Rigidity of 3-colorings of the d-dimensional discrete torus
Abstract:
We prove that a uniformly chosen proper coloring of Z_{2n}^d with 3 colors has a very rigid structure when the dimension d is sufficiently high. The coloring takes one color on almost all of either the even or the odd sub-lattice. In particular, one color appears on nearly half of the lattice sites. This model is the zero temperature case of the 3-states anti-ferromagnetic Potts model, which has been studied extensively in statistical mechanics. The proof involves results about graph homomorphisms and various combinatorial methods, and follows a topological intuition. Joint work with Ron Peled.
Schreiber Building Room 309 at 14:30.

Monday, November 14

No seminar due to nearby conference.

Monday, November 21

Ariel Yadin, Ben-Gurion University
Super-critical self avoiding walk is space filling
Abstract:
The Self Avoiding Walk (SAW) is a model proposed by Flory in the 50's to study polymer chains. The models studied basically refer to choosing a self avoiding path, P, in a lattice, with probability proportional to x^|P| , where |P| is the length of P. Here x is some positive real parameter. There is a conjectured phase transition of the behavior of a typical path according to whether x is above or below the "connective constant" of the lattice. The sub-critical case has been studied by Ioffe.
In dimension 2, it is conjectured that the critical case should converge to SLE(8/3). Resolving this conjecture is a major open problem in probability. In fact not much is known about geometric properties of the 2D SAW. The super-critical SAW is expected to be space-filling, and in dimension 2 to have the scaling limit SLE(8), which is a space filling curve.
In joint work with Gady Kozma and Hugo Duminil-Copin we show that super-critical SAW is space filling. The proof is fairly natural and uses a renormalization idea. The proof works for different lattices and all dimensions > 1.
We will not assume prior knowledge of SAW or SLE.
Schreiber Building Room 309 at 14:30.

Monday, November 28

Doron Puder, The Hebrew University of Jerusalem
Uniform Words are Primitive
Abstract:
Let a,b,c,... in S_n be random permutations on n elements, chosen at uniform distribution. What is the distribution of the permutation obtained by a fixed word in the letters a,b,c,..., such as ab,a^2, a^2bc^2b, or aba^(-2)b^(-1)? More concretely, do these new random permutations have uniform distribution? In general, a free word w in the free group F_k is called uniform if for every finite group G, the word map $w: G^k \to G$ induces uniform distribution on G (given uniform distribution on G^k). So which words are uniform?
This question is strongly connected to the notion of primitive words in the free group F_k. The word w is called primitive if it belongs to some basis, i.e. a free generating set. It is an easy observation that a primitive word is uniform. It was conjectured that the converse is also true. We prove it for F_2, and in a recent joint work with O. Parzanchevski, we manage to prove the conjecture in full. A key ingredient of the proofs is a new algorithm to detect primitive elements.
Schreiber Building Room 309 at 14:30.

Monday, December 5

Mike Hochman, The Hebrew University of Jerusalem
Equidistribution from fractals
Abstract:
By a classical result of Cassels and W. Schmidt, the standard middle-third Cantor sets contains normal numbers in any base which is not a power of 3 (a number is normal in base n if it equidistributes under the times-n mod 1 map). There are many extensions of this problem, e.g. Host's theorem about more general times-3 invariant measures, and results of Kauffman showing that there are many numbers with partial bounded quotients which are normal in every base.
I will discuss a new geometric/dynamical method with which one can prove results of this kind. The method has many advantages over the analytical approach which is usually used, for example it is robust when the target set is smoothly perturbed. I will also describe applications to some of the open problems in the field. This is joint work with Pablo Shmerkin.
Schreiber Building Room 309 at 14:30.

Double seminar! Note special time! Each talk will be 50 minutes.
Monday, December 12

Alexandre Stauffer, Microsoft Research
Tom Ellis, Tel Aviv University
Stauffer title: Space-time percolation and detection by mobile nodes.
Stauffer Abstract:
Consider a Poisson point process of intensity lambda in R^d. We denote the points as nodes and let each node move as an independent Brownian motion. Consider a target particle that is initially placed at the origin at time 0 and can move according to any continuous function. We say that the target is detected at time t if there exists at least one node within distance 1 of the target at time t. We show that if lambda is sufficiently large, the target will eventually be detected even if its motion can depend on the past, present and future positions of the nodes. In the proof we use coupling and multi-scale analysis to model this event as a fractal percolation process and show that a good event percolates in space and time.
Ellis title: The Brownian web is a two dimensional black noise.
Ellis Abstract:
The Brownian web is a stochastic process constructed from Brownian motions, and was one of the first known examples of a "black noise" in the sense of Tsirelson. I will discuss what it means to be a black noise, and demonstrate how the Brownian web is in fact a two-dimensional black noise. It is only the second known example of a two-dimensional black noise after Schramm and Smirnov's result on the scaling limit of critical planar percolation.
Joint work with Ohad Feldheim.
Schreiber Building Room 309 at 14:10.

Monday, December 19

Oren Louidor, University of California, Los Angeles
Trapping in the random conductance model.
We consider random walks on Z^d among nearest-neighbor random conductances which are i.i.d., positive, bounded uniformly from above but which can be arbitrarily close to zero. Our focus is on the detailed properties of the paths of the random walk conditioned to return back to the starting point after time 2n. We show that in the situations when the heat kernel exhibits subdiffusive behavior --- which is known to be possible in dimensions d ≥ 4-- the walk gets trapped for time of order n in a small spatial region. This proves that the strategy used to infer subdiffusive lower bounds on the heat kernel in earlier studies of this problem is in fact dominant. In addition, we settle a conjecture on the maximal possible subdiffusive decay in four dimensions and prove that anomalous decay is a tail and thus zero-one event.
Joint work with Marek Biskup, Alexander Vandenberg and Alexander Rozinov.
Schreiber Building Room 309 at 14:30.

Special Joint GAFA-Horowitz seminar! Note special place and time! Each talk will be 50 minutes.
Monday, December 26

Yuval Peres, Microsoft Research
Peter Mester, Hebrew University of Jerusalem
Peres title: Mixing times are hitting times of large sets.
Peres Abstract:
Consider a simple random walk on a connected, undirected, finite graph G. Mixing times and hitting times are fundamental parameters of the walk. We relate them by showing that the mixing time of a lazy walk is equivalent (up to a bounded factor) to the maximum over initial states x and large sets A of the hitting time of A starting from x. (Call a set “large” if its volume is at least 1/4 the volume of G). As an application, we show that the mixing time on a finite tree is robust to bounded change of edge conductances. It is an open problem whether the equivalence holds when we require the target set A to have volume at least Vol(G)/2. (joint work with Perla Sousi, University of Cambridge)
Mester title: A factor of i.i.d. with continuous marginals and infinite clusters spanned by identical labels.
Mester Abstract:
If (G,X,\mu) and (G, Y, \nu) are systems (where G is a group acting with measure preserving transformations on the probability spaces (X,\mu) and (Y,\nu)), then we say that the latter is a factor of the former if there is a map from X to Y which commutes with the group actions and the pushforward of \mu is \nu. We will deal with the case where both X and Y are [0,1]^G and the measure \mu is the product measure obtained by putting the Lebesgue measure on [0,1], factors of this system are called "factor of i.i.d.". In a more probabilistic language we may say that we label the group elements with i.i.d. uniform reals from [0,1] and then use some (deterministic) equivariant rule to "relabel" the group. In this context Gaboriau asked if the following is true: Assume that a factor of i.i.d. has the further property that the marginal labels are continuous (or equivalently uniform on [0,1]) and consider the process on a Cayley graph in the group. Does it follow that the clusters spanned by vertices of equal labels must be finite? We will present a construction which answers this question in the negative. The construction is a probabilistic one using simple facts about percolation on a tree.
Shenkar Building, Holzblat Hall 007 at 14:10.

Monday, January 2

Anita Winter, Universität Duisburg-Essen
The Aldous move on cladograms in the diffusion limit
Abstract:
A n-phylogenetic tree is a semi-labeled, unrooted and binary tree with n leaves labeled 1,2,...,n and with (n-2) unlabeled internal leaves and positive edge lengths representing the time spans between common ancestors. In biological systematics phylogenetic trees are used to represent the evolutionary relationship between species. If one does focus only on the kinship (that is taking all edge length of unit length), a more precise term is cladogram.
Aldous constructed a Markov chain on cladograms and gave bounds on their mixing time. On the other hand, Aldous also gave a notion of convergence of cladograms which shows that the uniform cladogram with N leaves and edge length re-scaled by a factor of 1/sqrt{N} converges to the so-called Continuum Random Tree (CRT). These two results suggest that if we re-scale edge lengths by a factor of 1/\sqrt{N} and speeding up time by a factor of N^{3/2} the Aldous move on cladograms converges in some sense to a continuous tree-valued diffusion. We will use Dirichlet form methods to construct limiting dynamics. (This is joint work with Leonid Mytnik, Technion Haifa)
Schreiber Building Room 309 at 14:30.

Monday, January 9

Boris Tsirelson, Tel Aviv University
Noise as a Boolean algebra of sigma-fields
Abstract:
The black noise of two-dimensional percolation, disclosed recently by Schramm, Smirnov and Garban, exceeds the limits of the existing framework based on one-dimensional intervals. I propose another framework --- Boolean algebras of sigma-fields. (See also http://arXiv.org/abs/1111.7270/)
Schreiber Building Room 309 at 14:30.

Monday, January 16

Subhroshekhar Ghosh, University of California, Berkeley
What does a Point Process Outside a Domain tell us about What's Inside?
Abstract:
In a Poisson point process we have independence between disjoint spatial domains, so the points outside a disk give us no information on the points inside. The story gets a lot more interesting for processes with stronger spatial correlation. In the case of Ginibre ensemble, a process arising from eigenvalues of random matrices, we prove that the outside points determine exactly the number of points inside, and further, we demonstrate that they determine nothing more. In the case of zero ensembles of Gaussian power series, we prove that the outside points determine exactly the number and the centre of mass of the inside points, and nothing further. These phenomena suggest a certain hierarchy of point processes according to their rigidity; Poisson, Ginibre and the Gaussian power series fit in at levels 0, 1 and 2 in this ladder. Time permitting, we will also look at some interesting consequences of our results, with applications to continuum percolation, reconstruction of Gaussian entire functions, and others. This is based on joint work with Fedor Nazarov, Yuval Peres and Mikhail Sodin.
Schreiber Building Room 309 at 14:30.

Monday, January 23

Gidi Amir, Bar-Ilan University
A continuum of exponents for the rate of escape of random walks on groups
Abstract:
For every 3/4 ≤ beta < 1 we construct a finitely generated group so that the expected distance of the simple random walk from its starting point after n steps is n^beta (up to constants). This answers a question of Vershik, Naor and Peres. In other examples, the speed exponent can fluctuate between any two values in this interval.
Previous examples were only of exponents of the form 1-1/2^k or 1, and were based on lamplighter (wreath product) constructions. (Other than the standard beta=1/2 and beta=1 which are simply diffusive and ballistic behaviours known for a wide variety of groups) In this lecture we will describe how a variation of the lamplighter construction, namely the permutational wreath product, can be used to get precise bounds on the rate of escape in terms of return probabilities of the random walk on some Schreier graphs. We will then show how groups of automorphisms of rooted trees, related to automata groups, can then be constructed and analyzed to get the desired rate of escape. This is joint work with Balint Virag of the University of Toronto. No previous knowledge of automaton groups or wreath products is assumed.
Schreiber Building Room 309 at 14:30.

Monday, January 30

Omer Bobrowski, Technion
Distance functions, critical points and topology for some random complexes
Abstract:
In this talk we focus on the distance function from a random set of points P in the Euclidean space. The distance function is continuous, however, it is not everywhere differentiable. Nevertheless, one can accurately define critical points and then apply Morse theory to it.
We study the number of critical points in small neighborhoods around P. Specifically, we are interested in the case where the number of points in P goes to infinity, and the size of the neighborhoods goes to zero. We present limit theorems for the number of critical points and show that it exhibits a phase transition, depending on how fast the size of the neighborhoods goes to zero. A similar phase transition was presented recently by Kahle and Meckes who studied the Betti-numbers of random geometric complexes. We show that this is more than just a coincidence, and discuss the connection between the distance function and geometric complexes.
Schreiber Building Room 309 at 14:30.





Spring Semester 2011


Monday, February 21

Manjunath Krishnapur, Indian Institute of Science, Bangalore
The single ring theorem
Abstract:
What are the eigenvalues of a typical matrix with a given set of singular values? We make the question precise by considering a random matrix of the form A=UDV, where D is a diagonal matrix and U,V are independent unitary matrices sampled from Haar measure.Under certain assumptions on the distribution of D, we show that A has a limiting spectral distribution, and characterize its properties. In particular, it has the surprising feature that the support of the limit spectral distribution is a connected annulus. This is joint work with Alice Guionnet and Ofer Zeitouni.
Schreiber Building Room 309 at 14:30.

Monday, February 28

Tom Ellis, University of Cambridge
From Diffusion Limited Aggregation to the Brownian Web via Conformal Mappings
Abstract:
Diffusion Limited Aggregation (DLA) is a planar growth model where the rate of growth at at point on the boundary of a cluster is proportional to the harmonic measure there. I will define the Hastings-Levitov growth models which approximate DLA, and show how they use ideas of conformal invariance which are analogous to those from the celebrated area of Schramm-Loewner Evolution.
I will demonstrate how the evolution of the harmonic measure on the cluster boundary corresponds to a stochastic flow, and will explain why -- in a suitable scaling limit -- the flow converges to an independent system of coalescing Brownion motions, known as the Brownian web.
Schreiber Building Room 309 at 14:30.

Monday, March 7

Nati Linial, Hebrew University
Probability and topology? What a strange combination
Abstract:
In this talk I review several recent attempts at introducing tools from the probabilistic method to the study of certain topological structures. This includes joint papers and ongoing work with: Roy Meshulam, Mishael Rosenthal, Tahl Nowik, Lior Aronshtam and Tomasz Luczak.
Schreiber Building Room 309 at 14:30.

Monday, March 14

Michael Bromberg, Tel Aviv University
Weak invariance principle for the local times of partial sums of Markov Chains
Abstract
Schreiber Building Room 209 at 14:30.
NOTE SPECIAL ROOM!

Monday, March 21

Omri Sarig, Weizmann Institute and Pennsylvania State University
Symbolic dynamics for C^{1+epsilon} surface diffeomorphisms with positive topological entropy
Abstract:
Suppose f:M -->M is a C^{1+epsilon} diffeomorphism of a compact smooth manifold of dimension two with topological entropy h>0. For every 0<delta<h, we construct a "delta-large" invariant set E such that f restricted to E has a countable Markov partition. It follows that f|E is a finite-to-one factor of a topological Markov shift. "Delta-large" means that E has full measure for every ergodic invariant measure with entropy bigger than delta. There are many consequences, for example -- every ergodic measure of maximal entropy is a finite-to-one factor of a positive recurrent countable Markov chain, and is therefore isomorphic to a Bernoulli scheme times a rotation.
Schreiber Building Room 309 at 14:30.

Monday, March 28

Elon Lindenstrauss, Hebrew University
Orbit closures for some higher rank Abelian actions
Abstract:
In 1967 Furstenberg discovered that any x2, x3 closed invariant subset of R/Z is closed or finite, contrasting with the situation for sets invariant under a single endomorphism. Even earlier, Cassels and Swinnerton-Dyer have made a deep conjecture on products of linear forms that is equivalent to a rigidity statement about orbit closures of the diagonal group in SL(n,R)/SL(n,Z) (namely that they are compact iff the orbit is periodic). However, the exact classification of orbit closures can be quite tricky, even on the conjectural level. I will present some positive and negative results in this direction based mostly on joint works with U. Shapira and Z. Wang.
Schreiber Building Room 309 at 14:30.

Monday, April 4

Johan Tykesson, Weizmann Institute
Percolation in the vacant set of Poisson cylinders
Abstract:
We consider a Poisson point process on the space of lines in R^d, where a multiplicative factor u>0 of the intensity measure determines the density of lines. Each line in the process is taken as the axis of a bi-infinite cylinder of radius 1. We investigate percolative properties of the vacant set, defined as the subset of R^d that is not covered by any such cylinder. We show that in dimensions d >= 4, there is a critical value u_*(d) \in (0,\infty), such that with probability 1, the vacant set has an unbounded component if u<u_*(d), and only bounded components if u>u_*(d). For d=3, we prove that the vacant set does not percolate for large u and that the vacant set intersected with a two-dimensional subspace of R^d does not even percolate for small u>0. This is joint work with David Windisch.
Schreiber Building Room 210 at 14:30.
NOTE SPECIAL ROOM!

Monday, April 11

Haya Kaspi, Technion
Stochastic PDE limits of many servers queues
Abstract
Schreiber Building Room 309 at 14:30.

Monday, April 18

Passover break - No seminar.

Monday, April 25

Passover break - No seminar.

Monday, May 2

Eviatar Procaccia, Weizmann Institute
Geometry of the Random interlacement
Abstract:
We consider the geometry of random interlacements on the $d$-dimensional lattice. We use ideas from stochastic dimension theory proved in \cite{benjamini2004geometry} to prove the following: Given that two vertices $x,y$ belong to the interlacement set, it is possible to find a path between $x$ and $y$ contained in the trace left by at most $\lceil d/2 \rceil$ trajectories. Moreover, this result is sharp in the sense that there are pairs of points in the interlacement set which cannot be connected by a path using the traces of at most $\lceil d/2 \rceil-1$ trajectories.
Schreiber Building Room 309 at 14:30.

Monday, May 9

Memorial day - No seminar.

Monday, May 16

Ran Tessler, Hebrew University
Zero Temperature Spin Glass Models
Abstract:
In this introductory talk we describe the Edwards-Anderson-Ising Spin Glass model. We consider ground states of this model on several types of graphs. In addition we describe a new dynamical process, which is a natural generalization of the famous Glauber dynamics (in zero temperature) whose limits are supported on the set of ground-states.
Joint work with Noam Berger.
Schreiber Building Room 309 at 14:30.

Monday, May 23

Joanna Kulaga, Nicolaus Copernicus University, Torun, Poland
Joining primeness property of order n
Abstract:
Joining primeness property of order n (JP(n)) is a generalization of such well-known properties as e.g. minimal self joining or simplicity. JP(n) property is also connected with singularity of the so-called maximal spectral type. On the other end of the "map" of possible dynamical systems lies a class of infinitely divisible systems. During my talk I will talk about the relation between these notions. As a byproduct I sketch a simple proof of Girsanov's theorem about spectral multiplicity function for Gaussian systems.
Schreiber Building Room 309 at 14:30.

Monday, May 30

Boris Solomyak, University of Washington
Tiling dynamical systems with an infinite invariant measure
Abstract:
We investigate non-primitive, non-minimal tiling substitutions in R^d, and characterize all "natural" invariant measures for the corresponding dynamical systems. In many cases, such an invariant measure is unique in some sense, and it is infinite (but sigma-finite). The first such example, in 1 dimension ("integer Cantor set") was considered by Alby Fisher. Our examples include the "Sierpinski gasket" and "Sierpinski carpet" tilings in the plane. (Joint work with Maria Isabel Cortez.)
Schreiber Building Room 309 at 14:30.

Monday, June 6

Isaac Meilijson, Tel Aviv University
Granule membranes play dice
Abstract:
Granules are secreted by the cell in either spontaneous or evoked mode, and the distribution of their volume is recorded, which we have modeled respectively as the exit and stationary distributions of a Growth & Elimination Markovian stochastic model. Growth requires granule-granule fusion and elimination requires granule-membrane fusion, by linking SNARE protein pairs diffusing in the granule and membrane surfaces. Transition rates of the Growth & Elimination model depend on the required number of such pairs for fusion, that can be identified (theoretically and experimentally estimated) from the two secretion volume distributions. Based on joint work with Ilan Hammel and Eyal Nitzany.
Schreiber Building Room 209 at 14:30.
NOTE SPECIAL ROOM!





Fall Semester 2010


Monday, October 18

Ron Peled, Tel Aviv University
An Introduction to Statistical Mechanics and Phase Transition Phenomenon
Abstract:
We will give an introduction to several basic models in statistical mechanics, including percolation, Ising and random surface models, and explain the phase transitions they exhibit. No previous knowledge in statistical mechanics will be assumed.
Schreiber Building Room 209 at 14:30.

Monday, October 25

Gady Kozma, Weizmann Institute
Random walks on the symmetric group
Abstract:
The group of all permutations of n elements is perhaps the most studied finite non-abelian group, in terms of random walk on it. An exciting aspect of the field is that every question can be approached probabilistically or algebraically, and people reached impressive results both using and not using its theory of representations. We will survey old and new results, and connections to the quantum Heisenberg ferromagnet. No prior knowledge of representation theory (or physics) will be assumed.
Schreiber Building Room 209 at 14:30.

Monday, November 1

Nicholas Crawford, Technion
A Curious Example of Localization via Randomness in Classical Statistical Mechanics
Abstract
Schreiber Building Room 309 at 14:30.

Monday, November 8

Senya Shlosman, Centre de Physique Théorique, Marseille, France
Understanding the arctic circle in the six-vertex model, and related open questions
Abstract:
I will review some properties of the six-vertex model with domain-wall boundary conditions, its relation with alternating sign matrices and totally symmetric plane partitions. I will then present some results concerning the arctic circle properties in this model (joint results with Pavel Bleher). Finally I will formulate some open questions concerning the behavior of non-intersecting random walks on the plane.
Schreiber Building Room 309 at 14:30.

Monday, November 15

Barak Weiss, Ben-Gurion University
On separated nets
Abstract:
A separated net is a uniformly discrete subset of R^d, i.e. any point in R^d is bounded distance from a point in the set, and there is a lower bound on distance of points in the set from each other. One way to construct dynamical separated nets is to consider return times to a section of a minimal R^d-flow on a compact space. Gromov asked whether any separated net is bilipschitz to Z^d, and a counterexample was given in 1998 by Burago, Kleiner and McMullen. However for natural dynamical separated nets, it is in general a hard question to decide whether they are bilipschitz to Z^d. I will present some recent results on dynamical separated nets, joint with Alan Haynes and Yaar Solomon.
Schreiber Building Room 309 at 14:30.

Monday, November 22

Ofer Zeitouni, Weizmann Institute
From branching random walks to Gaussian Free Fields
Abstract:
The (discrete) Gaussian Free Field on a finite graph is the process {X_v} with density proportional to e^{-\sum_{v~w} (X_v-X_w)^2}; it has played an important role in many aspects of contemporary probability theory, as well as in mathematical physics through models for random interfaces and quantum gravity. A natural question relates to the magnitude of fluctuations of the maximum of the field; the planar case, where the graph is a box of side N, is of particular interest.
Branching random walks (BRW's) model the (spatial) evolution of a population, where particles split and then perform independent random motion (on R). As was shown by Bramson in the late 1970's, the behavior of the outliers of the population (ie, the particles that moved farthest from the starting point, after n generations) is determined, in the Gaussian displacement case, by solutions of the Kolmogorov-Petrovsky-Piscounov equation. In the non-Gaussian case, a proof that fluctuations of the maximum are of order 1 was given only very recently.
I will describe surprising links that BRW's have with Gaussian free fields, first passage percolation, and the cover time of graphs by random walks; I will then explain how arguments developed for the study of both Gaussian and non-Gaussian BRW's played a role in a recent resolution of the conjecture that fluctuations of the maximum of the Gaussian Free Field in dimension 2 are bounded.
Based on joint works with E. Bolthausen, J.-D. Deuschel and with M. Bramson
Schreiber Building Room 309 at 14:30.

Monday, November 29

Svetlana Katok, Penn State University
Structure of attractors for (a,b)-continued fraction transformations
Abstract:
I will discuss a two-parameter family of one-dimensional maps related to so-called (a,b)-continued fractions and studied jointly with Ilie Ugarcovici. The associated natural extension maps have attractors with finite rectangular structure for the entire parameter set except for a Cantor-like set of one-dimensional Lebesgue zero measure on the boundary that we completely described. The structure of these attractors can be “computed” from the data (a, b), and for a dense open set of parameters the Reduction theory conjecture holds, i.e. every point is mapped to the attractor after finitely many iterations. If time permits, I will explain how this theory can be applied to the study of ergodic properties of associated Gauss-like maps and for coding of geodesics on the modular surface.
Schreiber Building Room 309 at 14:30.

Monday, December 6

Anatole Katok, Penn State University
Absolutely continuous invariant measure for commuting maps and flows
Abstract:
The only situation In traditional dynamics where an open set of a system admits an absolutely continuous invariant measure appears for expanding maps of the circle. For invertible systems, i.e. diffeomorphisms and smooth flows, existence of such a measure is at best an infinite codimension condition. For smooth systems with multi-dimensional time, i.e. commuting maps or vector fields, the situation changes dramatically. Here certain dynamical properties of an invariant measure involving Lyapunov characteristic exponents and entropy imply absolute continuity. As an application certain global topological conditions on an action, such as homotopy types of its elements, imply existence of an absolutely continuous invariant measure.
Schreiber Building Room 309 at 14:30.

Monday, December 13

Ori Gurel-Gurevich, University of British Columbia, Vancouver, Canada
Poisson Thickening
Abstract:
Can a Poisson process be thickened? That is, can more points be added deterministically to a Poisson process, so that the resulting process is also a Poisson process (of higher intensity)? We will show that this can be done, but not equivariantly (i.e. not in a way that commutes with some shift).
In recent years, there has been much interest in problems of this kind: given a stochastic spatial process X, can it be extended to another process Y (perhaps under additional constraints)? For example, can the cells of a Poisson-Voronoi tessellation be colored deterministically and equivariantly, such that adjacent cells have different colors?
We will survey results of this kind, with particular emphasis on those which yield pretty pictures and explain the solution to the thickening problem in some detail.
Joint Work with Ron Peled.
Schreiber Building Room 309 at 14:30.

Monday, December 20

Yuval Peres, Microsoft Research
Brownian motion with variable drift and the Pascal principle
Abstract:
It is well known that Brownian motion on the line has no isolated zeros almost surely; Does this hold if we add a deterministic variable drift?
Similarly, we know that planar Brownian motion has paths of zero area almost surely; does this hold when adding a variable drift? Does the expected volume of the Wiener sausage increase when we add drift?
We will answer these questions and relate the last one to random geometric graphs, rearrangement inequalities and the “Pascal principle”.
(Talk based on joint works with T. Antunovic, J. Miller, J. Ruscher, P. Sousi, A. Stauffer, A. Sinclair and B. Vermesi).
Schreiber Building Room 309 at 14:30.

Monday, December 27

Eitan Bachmat, Ben-Gurion University
A symmetry satisfied by the Pollaczek - Khinchine formula
Abstract:
Queueing theory is an important branch of applied probability. A fundamental result in queueing theory is the Pollaczek-Khinchine formula (1930, 1932) for the average waiting time in a single server queue with Poisson arrivals. We provide new insight regarding this old formula by showing that it has a certain symmetry/duality. Interestingly, this type of symmetry first appeared in a simple computation of Riemann (1859). The symmetry is useful in the analysis of queues which are typical of supermarkets or minimarkets. If time permits we will explain how these might be improved in practice, saving! a few precious minutes of our lives.
Schreiber Building Room 309 at 14:30.

Monday, January 3

Hugo Duminil-Copin, Université de Genève
Critical temperature of the square lattice Potts model
Abstract
Schreiber Building Room 309 at 14:30.

Monday, January 10

Ivan Corwin, Courant Institute, New York University
Beyond the Gaussian Universality Class
Abstract:
The Gaussian central limit theorem says that for a wide class of stochastic systems, the bell curve (Gaussian distribution) describes the statistics for random fluctuations of important observables. In this talk I will look beyond this class of systems to a collection of probabilistic models which include random growth models, polymers, particle systems, matrices and stochastic PDEs, as well as certain asymptotic problems in combinatorics and representation theory. I will explain in what ways these different examples all fall into a single new universality class with a much richer mathematical structure than that of the Gaussian.
Schreiber Building Room 309 at 14:30.

Monday, January 17

David Ralston, Ben-Gurion University
Heavy Sets: Structure and Dimension
Abstract:
Given a measure preserving transformation on a space and a function of zero mean, the heavy set is the set of points whose ergodic sums remain nonnegative for all (forward) time. After discussing a few generic results, we will turn our attention to the specific setting where the transformation is an irrational circle rotation and the function is the characteristic function of the interval [0,1/2]. In this setting, we will show that the heavy set is almost-surely (with regard to the rotation parameter) the union of a perfect Cantor set of both Hausdorff and box dimension c, where c is some constant strictly between zero and one, together with countably many isolated points.
Schreiber Building Room 309 at 14:30.




Past years
School of Mathematical Sciences