Thierry de la Rue
Title: Krengel vs. Poisson entropy for infinite measure preserving transformations

Abstract: Several notions of entropy for infinite measure preserving transformations have been proposed, which elegantly generalize Kolmogorov's entropy of a probability-preserving transformation, in particular by Krengel (1967) and Parry (1969). More recently, Emmanuel Roy (2005) suggested to consider the so-called Poisson entropy, which is defined as the Kolmogorov entropy of the Poisson suspension of the transformation. These three notions of entropy coincide in many known cases, and it was an open question to decide whether they were always equal. In this talk I will present a recent counterexample showing that Poisson entropy can be positive while Krengel entropy vanishes.