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Register a new userMultiple recurrence and large intersections for abelian group actions
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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.
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We establish characteristic factors for natural classes of polynomial multiple ergodic averages in rings of integers and derive corresponding Khintchine-type recurrence theorems, extending results of Frantzikinakis and Kra and of Frantzikinakis about polynomial configurations in $mathbb{Z}$. Using previous work of Griesmer and of the second author and Robertson, we reduce the problem of finding characteristic factors to proving a result on equidistribution of polynomial orbits in nilmanifolds, which is of independent interest.
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