Pointwise ergodic theorem for locally countable quasi-pmp graphs

TL;DR

This paper establishes a pointwise ergodic theorem for locally countable quasi-pmp graphs, extending classical results to a broader setting and introducing new methods for analyzing hyperfiniteness and ergodic properties.

Contribution

It provides a self-contained proof of the ergodic theorem for quasi-pmp graphs, generalizing previous pmp results and introducing novel graph invariants and the concept of visibility.

Findings

Every ergodic locally countable quasi-pmp graph contains an ergodic hyperfinite subgraph.

Every ergodic treeable equivalence relation has an ergodic hyperfinite free factor.

The new methods apply more broadly than previous probability-based proofs.

Abstract

We prove a pointwise ergodic theorem for quasi-probability-measure-preserving (quasi-pmp) locally countable measurable graphs, equivalently, Schreier graphs of quasi-pmp actions of countable groups. For ergodic graphs, the theorem gives an increasing sequence of Borel subgraphs with finite connected components over which the averages of any $L^1$ function converges to its expectation. This implies that every (not necessarily pmp) locally countable ergodic Borel graph on a standard probability space contains an ergodic hyperfinite subgraph. A consequence of this is that every ergodic treeable equivalence relation has an ergodic hyperfinite free factor. The pmp case of the main theorem was first proven by R. Tucker-Drob using a deep result from probability theory. Our proof is different: it is self-contained and applies more generally to quasi-pmp graphs. Among other things, it involves introducing a graph invariant concerning asymptotic averages of functions and a method of tiling a large part of the space with finite sets with prescribed properties. The non-pmp setting additionally exploits a new quasi-order called visibility to analyze the interplay between the Radon--Nikodym cocycle and the graph structure, providing a sufficient condition for hyperfiniteness.