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arXiv 1208.0655math.DSmath.GR
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可数群的混合作用几乎是自由的

Mixing actions of countable groups are almost free

Robin Tucker-Drob

摘要:

可数无限群Γ的保测度作用称为全遍历的,如果Γ的每个无限子群作用遍历.例如,所有混合和轻度混合动作都是完全遍历的。本文证明了若Γ的作用是全遍历的,则存在Γ的有限正规子群N,使得几乎每一点的稳定子都等于N.令人惊讶的证明依赖于组理论的事实(证明霍尔和Kulatilaka以及Kargapolov),每一个无限的地方有限群包含一个无限的阿贝尔子群,其中所有已知的证明依赖于费特-汤普森定理。因此,我们推出了可数群的群论特征,其非平凡伯努利因子都是免费的:这些正是没有有限正规子群以外的其他平凡子群的群体。

英文摘要:

A measure preserving action of a countably infinite group Γis called totally ergodic if every infinite subgroup of Γacts ergodically. For example, all mixing and mildly mixing actions are totally ergodic. This note shows that if an action of Γis totally ergodic then there exists a finite normal subgroup N of Γsuch that the stabilizer of almost every point is equal to N. Surprisingly the proof relies on the group theoretic fact (proved by Hall and Kulatilaka as well as by Kargapolov) that every infinite locally finite group contains an infinite abelian subgroup, of which all known proofs rely on the Feit-Thompson theorem. As a consequence we deduce a group theoretic characterization of countable groups whose non-trivial Bernoulli factors are all free: these are precisely the groups that posses no finite normal subgroup other than the trivial subgroup.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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