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arXiv 0905.2278math.DS
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可数Amenable Groups中的和集现象

Sumset Phenomenon in Countable Amenable Groups

Mathias Beiglboeck, Vitaly Bergelson, Alexander Fish

摘要:

Jin证明了:当$A$和$B$是$\Z$中的正上密度集时,$A+B$是分段联会的. Jin和Keisler随后将Jin的定理推广到某个阿贝尔群族,其中特别包含$\Z^d$。通过讨论Jin和Keisler的一个问题,我们证明了这个结果可以推广到可数顺从群.此外,我们建立这样的和集(或-取决于符号-“productsets”)是分段玻尔,一个结果,为$G=\Z$证明了Bergelson,Furstenberg和Weiss。在阿贝尔群$G$的情况下,我们证明了一个集合是分片玻尔的当且仅当它包含两个正的上Banach密度集的和集。

英文摘要:

Jin proved that whenever $A$ and $B$ are sets of positive upper density in $\Z$, $A+B$ is piecewise syndetic. Jin's theorem was subsequently generalized by Jin and Keisler to a certain family of abelian groups, which in particular contains $\Z^d$. Answering a question of Jin and Keisler, we show that this result can be extended to countable amenable groups. Moreover we establish that such sumsets (or -- depending on the notation -- "productsets") are piecewise Bohr, a result which for $G=\Z$ was proved by Bergelson, Furstenberg and Weiss. In the case of an abelian group $G$, we show that a set is piecewise Bohr if and only if it contains a sumset of two sets of positive upper Banach density.

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