Ore localization of amenable monoid actions and applications towards entropy $-$ addition formulas and the bridge theorem

TL;DR

This paper develops a framework for Ore localization and colocalization of monoid actions, preserving entropy, and extends addition formulas and the bridge theorem to broader classes of monoid actions on groups.

Contribution

It introduces Ore localization and colocalization for monoid actions, preserving entropy, and extends the Addition Theorem and Bridge Theorem to cancellative right amenable monoids.

Findings

Entropy is preserved under Ore localization and colocalization.

Extended the Addition Theorem for topological entropy to monoid actions on compact groups.

Established the Bridge Theorem relating topological and algebraic entropy for Abelian groups.

Abstract

For a left action $S\overset{\lambda}{\curvearrowright}X$ of a cancellative right amenable monoid $S$ on a discrete Abelian group $X$, we construct its Ore localization $G\overset{\lambda^*}{\curvearrowright}X^*$, where $G$ is the group of left fractions of $S$; analogously, for a right action $K\overset{\rho}\curvearrowleft S$ on a compact space $K$, we construct its Ore colocalization $K^*\overset{\rho^*}{\curvearrowleft} G$. Both constructions preserve entropy, i.e., for the algebraic entropy $h_{\mathrm{alg}}$ and for the topological entropy $h_{\mathrm{top}}$ one has $h_{\mathrm{alg}}(\lambda)=h_{\mathrm{alg}}(\lambda^*)$ and $h_{\mathrm{top}}(\rho)=h_{\mathrm{top}}(\rho^*)$, respectively. Exploiting these constructions and the theory of quasi-tilings, we extend the Addition Theorem for $h_{\mathrm{top}}$, known for right actions of countable amenable groups on compact metrizable groups, to right actions $K\overset{\rho}{\curvearrowleft} S$ of cancellative right amenable monoids $S$ (with no restrictions on the cardinality) on arbitrary compact groups $K$. When the compact group $K$ is Abelian, we prove that $h_{\mathrm{top}}(\rho)$ coincides with $h_{\mathrm{alg}}(\hat{\rho})$, where $S\overset{\hat{\rho}}\curvearrowright X$ is the dual left action on the discrete Pontryagin dual $X=\hat{K}$, that is, a so-called Bridge Theorem. From the Addition Theorem for $h_{\mathrm{top}}$ and the Bridge Theorem, we obtain an Addition Theorem for $h_{\mathrm{alg}}$ for left actions $S\overset{\lambda}\curvearrowright X$ on discrete Abelian groups, so far known only under the hypotheses that either $X$ is torsion or $S$ is locally monotileable. The proofs substantially use the unified approach towards entropy based on the entropy of actions of cancellative right amenable monoids on appropriately defined normed monoids.