Ordered structures and large conjugacy classes

Aleksandra Kwiatkowska; Maciej Malicki

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arXiv · 1903.00936

Ordered structures and large conjugacy classes

Aleksandra Kwiatkowska·Maciej Malicki

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Abstract

This article is a contribution to the following problem: does there exist a Polish non-archimedean group (equivalently: automorphism group of a Fraisse limit) that is extremely amenable, and has ample generics. As Fraisse limits whose automorphism groups are extremely amenable must be ordered, i.e., equipped with a linear ordering, we focus on ordered Fraisse limits. We prove that automorphism groups of the universal ordered boron tree, and the universal ordered poset have a comeager conjugacy class but no comeager $2$-dimensional diagonal conjugacy class. We also provide general conditions implying that there is no comeager conjugacy class, comeager $2$-dimensional diagonal conjugacy class or non-meager $2$-dimensional topological similarity class in the automorphism group of an ordered Fraisse limit. We provide a number of applications of these results.

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BibTeX RIS

Aleksandra Kwiatkowska, Maciej Malicki. 2019-03-03. Ordered structures and large conjugacy classes. https://arxiv.org/abs/1903.00936

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