Geometric amenability in totally disconnected locally compact groups

TL;DR

This paper explores the connection between geometric properties of groups and graphs, providing new proofs and extending the concept of amenability to totally disconnected locally compact groups using geometric and Lipschitz action perspectives.

Contribution

It offers a geometric proof of a key amenability characterization for graphs and groups, and introduces the notion of uniform geometric non-amenability for locally compact groups.

Findings

A quasitransitive graph is amenable iff its automorphism group is amenable and unimodular.

Proper Lipschitz actions on amenable graphs imply group amenability and unimodularity.

Introduction of uniform geometric non-amenability and its relation to group actions and modular homomorphism.

Abstract

We give a short geometric proof of a result of Soardi & Woess and Salvatori that a quasitransitive graph is amenable if and only if its automorphism group is amenable and unimodular. We also strengthen one direction of that result by showing that if a compactly generated totally disconnected locally compact group admits a proper Lipschitz action on a bounded-degree amenable graph then that group is amenable and unimodular. We pass via the notion of geometric amenability of a locally compact group, which has previously been studied by the second author and is defined by analogy with amenability, only using right Folner sets instead of left Folner sets. We also introduce a notion of uniform geometric non-amenability of a locally compact group, and relate this notion in various ways to actions of that group on graphs and to its modular homomorphism.