Behaviour of entropy under bounded and integrable orbit equivalence

TL;DR

This paper investigates how entropy behaves under specific types of orbit equivalence between actions of finitely generated amenable groups, establishing a proportionality relation under tail-bound conditions.

Contribution

It introduces and analyzes integrable and bounded stable orbit equivalences, deriving a formula linking entropy ratios to the compression constant, and shows entropy invariance under full orbit equivalence with tail bounds.

Findings

Entropy ratios are proportional to the compression constant.

Entropy is invariant under full orbit equivalence with tail bounds.

New notion of weighted cost based on group element lengths.

Abstract

Let $G$ and $H$ be infinite finitely generated amenable groups. This paper studies two notions of equivalence between actions of such groups on standard Borel probability spaces. They are defined as stable orbit equivalences in which the associated cocycles satisfy certain tail bounds. In `integrable stable orbit equivalence', the length in $H$ of the cocycle-image of an element of $G$ must have finite integral over its domain (a subset of the $G$-system), and similarly for the reverse cocycle. In `bounded stable orbit equivalence', these functions must be essentially bounded in terms of the length in $G$. `Integrable' stable orbit equivalence arises naturally in the study of integrable measure equivalence of groups themselves, as introduced recently by Bader, Furman and Sauer. The main result is a formula relating the Kolmogorov--Sinai entropies of two actions which are equivalent in one of these ways. Under either of these tail assumptions, the entropies stand in a proportion given by the compression constant of the stable orbit equivalence. In particular, in the case of full orbit equivalence subject to such a tail bound, entropy is an invariant. This contrasts with the case of unrestricted orbit equivalence, under which all free ergodic actions of countable amenable groups are equivalent. The proof uses an entropy-bound based on graphings for orbit equivalence relations, and in particular on a new notion of cost which is weighted by the word lengths of group elements.