Abstract: After introducing expander graphs and briefly discussing Lubotzky-Weiss
Independence Problem  for groups and expanders I will talk about  recent
developments pertaining to establishing the expansion property for congruence
quotients of thin groups --discrete subgroups of semi-simple groups which are
Zariski dense but of infinite index. This expansion property can be viewed as a
far-reaching generalization of the strong approximation theorem and has many
applications.

Coffee will be served at 12:00 before the lecture
at Schreiber building 006