Five remarks about random walks on groups

TL;DR

This paper provides new, concise proofs of classical and recent results on random walks on groups, including properties of the Poisson boundary, ergodicity, and harmonic functions, with applications to hyperbolic groups and combinatorics.

Contribution

It introduces simplified, self-contained proofs of key theorems in the theory of random walks on groups, and explores new structures like Besov spaces for harmonic functions.

Findings

Elementary proof of drift characterization of Liouville groups

New proof of ergodicity of the Poisson boundary

Alternative approach to harmonic functions on hyperbolic groups

Abstract

The main aim of the present set of notes is to give new, short and essentially self-contained proofs of some classical, as well as more recent, results about random walks on groups. For instance, we shall see that the drift characterization of Liouville groups, due to Kaimanovich-Vershik and Karlsson-Ledrappier (and to Varopoulos in some important special cases) admits a very short and quite elementary proof. Furthermore, we give a new, and rather short proof of (a weak version of) an observation of Kaimanovich (as well as a small strengthening thereof) that the Poisson boundary of any symmetric measured group $(G,\mu)$, is doubly ergodic, and the diagonal $G$-action on its product is ergodic with unitary coefficients. We also offer a characterization of weak mixing for ergodic $(G,\mu)$-spaces parallel to the measure-preserving case. We shed some new light on Nagaev's classical technique to prove central limit theorems for random walks on groups. In the interesting special case when the measured group admits a product current, we define a Besov space structure on the space of bounded harmonic functions with respect to which the the associated convolution operator is quasicompact without any assumptions on finite exponential moments. For Gromov hyperbolic measured groups, this gives an alternative proof of the fact that every H\"older continuous function with zero integral with respect to the unique stationary probability measure on the Gromov boundary is a co-boundary. Finally, we give a new and almost self-contained proof of a special case of a recent combinatorial result about piecewise syndeticity of product sets in groups by the author and A. Fish.