Abstract: This lecture is dedicated to the memory of Israel Gohberg, Z"L. The title was one of his favorite questions. Some answers will be provided in the setting of a class of reproducing kernel Hilbert spaces that was introduced and characterized by L. de Branges (with an important technical improvement by J. Rovnyak). Applications to interpolation problems of the Nevanlinna-Pick type will be discussed. The lecture will be expository.

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