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Preprint Number 1497

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1497. Tomasz Rzepecki
Bounded Invariant Equivalence Relations

Submission date: 11 October 2018


183 pages; PhD thesis
We study strong types and Galois groups in model theory from a topological and descriptive-set-theoretical point of view, leaning heavily on topological dynamical tools. More precisely, we give an abstract (not model theoretic) treatment of problems related to cardinality and Borel cardinality of strong types, quotients of definable groups and related objecets, generalising (and often improving) essentially all hitherto known results in this area. In particular, we show that under reasonable assumptions, strong type spaces are "locally" quotients of compact Polish groups. It follows that they are smooth if and only if they are type-definable, and that a quotient of a type-definable group by an analytic subgroup is either finite or of cardinality at least continuum.

Mathematics Subject Classification: 03C45, 54H05, 54H20, 22C05

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Full text arXiv 1810.05113: pdf, ps.

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