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arXiv 2102.07273math.DSmath.CO
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阿贝尔群的多重遍历平均与Khintchine型递归

Multiple ergodic averages in abelian groups and Khintchine type recurrence

Or Shalom

摘要:

设$G$是可数交换群。研究了某些$a,b\in\mathbb{Z}$的形式为$\{ag,bg,(a+b)g\}$的组态的遍历平均。在对$G$的一些假设下,我们证明了这些平均值的泛特征因子是$2$ -步幂零齐性空间的因子。作为应用,我们得到了一个Khintchine型递推结果.特别地,我们证明了:对于每个可数交换群$G$,如果$a,b\in\mathbb{Z}$使得$aG,bG,(b-a)G$和$(a+b)G$在$G$中是有限指数的,则对于每个$E\subset G$和$\varepsilon>0$,集合$$\{g\in G : d(E\cap E-ag\cap E-bg\cap E-(a+b)g)\geq d(E)^4-\varepsilon\}$$是联合的.这推广了Bergelson Host和Kra,Bergelson Tao和Ziegler以及作者分别对$G=\mathbb{Z}$,$G=\mathbb{F}_p^ω$和$G=\bigoplus_{p\in P}\mathbb{F}_p$的先前结果。

英文摘要:

Let $G$ be a countable abelian group. We study ergodic averages associated with configurations of the form $\{ag,bg,(a+b)g\}$ for some $a,b\in\mathbb{Z}$. Under some assumptions on $G$, we prove that the universal characteristic factor for these averages is a factor of a $2$-step nilpotent homogeneous space. As an application we derive a Khintchine type recurrence result. In particular, we prove that for every countable abelian group $G$, if $a,b\in\mathbb{Z}$ are such that $aG,bG,(b-a)G$ and $(a+b)G$ are of finite index in $G$, then for every $E\subset G$ and $\varepsilon>0$ the set $$\{g\in G : d(E\cap E-ag\cap E-bg\cap E-(a+b)g)\geq d(E)^4-\varepsilon\}$$ is syndetic. This generalizes previous results for $G=\mathbb{Z}$, $G=\mathbb{F}_p^ω$ and $G=\bigoplus_{p\in P}\mathbb{F}_p$ by Bergelson Host and Kra, Bergelson Tao and Ziegler and the author, respectively.

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