Fraïssé structures and a conjecture of Furstenberg

Abstract

We study problems concerning the Samuel compactification of the automorphism group of a countable first-order structure. A key motivating question is a problem of Furstenberg and a counter-conjecture by Pestov regarding the difference between $S(G)$, the Samuel compactification, and $E(M(G))$, the enveloping semigroup of the universal minimal flow. We resolve Furstenberg's problem for several automorphism groups and give a detailed study in the case of $G=S_{\infty }$, leading us to define and investigate several new types of ultrafilter on a countable set.

Formerly titled "Samuel compactifications of automorphism groups," the paper was accepted in Comment. Math. Univ. Carolin., and the title was changed at the request of the referee