Topological dynamics and the complexity of strong types

7 November 2017
Krzysztof Krupiński

The talk is based on my joint work with Anand Pillay and Tomasz Rzepecki.

I will describe some connections between various objects from topological dynamics associated with a given first order theory and various Galois groups of this theory. One of the main corollaries is a natural presentation of the closure of the neutral element of the Lascar Galois group of any given theory $T$ (this closure is a group sometimes denoted by $Gal_0(T)$) as a quotient of a compact Hausdorff group by a dense subgroup.

As an application, I will present a very general theorem concerning the complexity of bounded, invariant equivalence relations (whose classes are sometimes called strong types) in countable theories, generalizing a theorem of Kaplan, Miller and Simon concerning Borel cardinalities of Lascar strong types and also later extensions of this result to certain bounded, $F_\sigma$ equivalence relations (which were obtained in a paper of Kaplan and Miller and, independently, in a paper of Rzepecki and myself). The main point of our general theorem says that in a countable theory, any bounded, invariant equivalence relation defined
on the set of realizations of a single complete type over $\emptyset$ is type-definable if and only if it is smooth (in the sense of descriptive set theory). If time permits, I will very briefly mention more recent developments in this direction (also based on the results from the first paragraph) which will appear in my future paper with Rzepecki.