Multiple recurrence and large intersections for abelian group actions

TL;DR

This paper investigates large intersection phenomena in multiple recurrence for measure-preserving actions of countable abelian groups, extending previous results and providing new combinatorial applications.

Contribution

It generalizes multiple recurrence and large intersection results to broader classes of abelian group actions, including new conditions and applications.

Findings

Sets of recurrence are syndetic under certain homomorphism conditions.

Generalization of classical results from $\\mathbb{Z}$-actions to broader abelian groups.

New combinatorial applications via ergodic theory.

Abstract

The purpose of this paper is to study the phenomenon of large intersections in the framework of multiple recurrence for measure-preserving actions of countable abelian groups. Among other things, we show: (1) If $G$ is a countable abelian group and $\varphi, \psi : G \to G$ are homomorphisms such that $\varphi(G)$, $\psi(G)$, and $(\psi - \varphi)(G)$ have finite index in $G$, then for every ergodic measure-preserving system $(X, \mathcal{B}, \mu, (T_g)_{g \in G})$, every set $A \in \mathcal{B}$, and every $\varepsilon > 0$, the set $\{g \in G : \mu(A \cap T_{\varphi(g)}^{-1}A \cap T_{\psi(g)}^{-1}A) > \mu(A)^3 - \varepsilon\}$ is syndetic. (2) If $G$ is a countable abelian group and $r,s \in \mathbb{Z}$ are integers such that $rG$, $sG$, and $(r \pm s)G$ have finite index in $G$, then for every ergodic measure-preserving system $(X, \mathcal{B}, \mu, (T_g)_{g \in G})$, every set $A \in \mathcal{B}$, and every $\varepsilon > 0$, the set $\{g \in G : \mu(A \cap T_{rg}^{-1}A \cap T_{sg}^{-1}A \cap T_{(r+s)g}^{-1}A) > \mu(A)^4 - \varepsilon\}$ is syndetic. In particular, these extend and generalize results of Bergelson, Host, and Kra concerning $\mathbb{Z}$-actions and of Bergelson, Tao, and Ziegler concerning $\mathbb{F}_p^{\infty}$-actions. Using an ergodic version of the Furstenberg correspondence principle, we obtain new combinatorial applications. We also discuss numerous examples shedding light on the necessity of the various hypotheses above. Our results lead to a number of interesting questions and conjectures, formulated in the introduction and at the end of the paper.