Local weak$^{*}$-Convergence, algebraic actions, and a max-min principle

TL;DR

This paper introduces a new abstract method to analyze algebraic actions of sofic groups, establishing a max-min principle relating microstates and entropy, with applications to residually finite groups.

Contribution

It develops a novel approach using a limiting object with a poset structure to prove strong soficity and entropy equivalences in algebraic actions.

Findings

Established a max-min principle for algebraic actions with ergodic centralizer.

Proved equivalence of topological and measure entropy for residually finite groups.

Extended results to cases with non-ergodic centralizer.

Abstract

We continue our study of when topological and measure-theoretic entropy agree for algebraic action of sofic groups. Specifically, we provide a new abstract method to prove that an algebraic action is strongly sofic. The method is based on passing to a "limiting object" for sequences of measures which are asymptotically supported on topological microstates. This limiting object has a natural poset structure that we are able to exploit to prove a max-min principle: if the sofic approximation has ergodic centralizer, then the largest subgroup on which the action is a local weak$^{*}$-limit of measures supported on topological microstates is equal to the smallest subgroup which absorbs all topological microstates. We are able to provide a version for the case when the centralizer is not ergodic. We give many applications, including show that for residually finite groups completely positive (lower) topological entropy (in the presence) is equivalent to completely positive (lower) measure entropy in the presence.