Karma Dajani

Title: Random entropy of random group actions and recurrence of cocycles

We consider the (random) entropy of systems of random transformations, where the transformations are chosen from the set of generators of a ${\bf Z}^d $ action. Every such random group action is generated via a cocycle. We show that the classical definition gives unsatisfactory entropy results in the higher-dimensional case, i.e.\ when $d \geq 2$. We propose a new definition of the random entropy for random group actions which agrees with the classical definition in the one-dimensional case, and which gives satisfactory results in higher dimensions. We identify the random entropy by a concrete formula which makes it possible to compute the random entropy in certain cases. We also show that for a random group action the associated cocyle is recurrent if and only if the random entropy is zero. This implies a Polya type dichotomy for this situation