Multiple recurrence and large intersections for abelian group actions

Ethan Ackelsberg, Vitaly Bergelson, Andrew Best

Published 2021-01-08Version 1

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 $G$, and no quotient of $G$ is isomorphic to a Pr\"{u}fer $p$-group with $p \mid rs(s-r)$, 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, this extends and generalizes 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. Our results lead to a number of interesting questions and conjectures, formulated in the introduction and at the end of the paper.