Extremely amenable automorphism groups of countable structures
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arXiv
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| Auteurs principaux: | , , , |
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| Format: | Preprint |
| Publié: |
2024
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| _version_ | 1866917353765208064 |
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| author | Etedadialiabadi, Mahmood Gao, Su Li, Feng Li, Ruiwen |
| author_facet | Etedadialiabadi, Mahmood Gao, Su Li, Feng Li, Ruiwen |
| contents | In this paper we address the question: How many pairwise non-isomorphic extremely amenable groups are there which are separable metrizable or even Polish? We show that there are continuum many such groups. In fact we construct continuum many pairwise non-isomorphic extremely amenable groups as automorphism groups of countable structures. We also consider this classification problem from the point of view of descriptive set theory by showing that the class of all extremely amenable closed subgroups of $S_\infty$ is Borel and their isomorphism relation is more complex than any isomorphism relation of countable structures in the Borel reducibility hierarchy. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2411_02760 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Extremely amenable automorphism groups of countable structures Etedadialiabadi, Mahmood Gao, Su Li, Feng Li, Ruiwen Logic General Topology 03E15 22F50 In this paper we address the question: How many pairwise non-isomorphic extremely amenable groups are there which are separable metrizable or even Polish? We show that there are continuum many such groups. In fact we construct continuum many pairwise non-isomorphic extremely amenable groups as automorphism groups of countable structures. We also consider this classification problem from the point of view of descriptive set theory by showing that the class of all extremely amenable closed subgroups of $S_\infty$ is Borel and their isomorphism relation is more complex than any isomorphism relation of countable structures in the Borel reducibility hierarchy. |
| title | Extremely amenable automorphism groups of countable structures |
| topic | Logic General Topology 03E15 22F50 |
| url | https://arxiv.org/abs/2411.02760 |