Dense free subgroups of automorphism groups of homogeneous partially ordered sets

Abstract

A countable poset is ultrahomogeneous if every isomorphism between its finite subposets can be extended to an automorphism. The groups $\mathrm{Aut}(A)$ of such posets $A$ have a natural topology in which $\mathrm{Aut}(A)$ are Polish topological groups. We consider the problem whether $\mathrm{Aut}(A)$ contains a dense free subgroup of two generators. We show that if $A$ is ultrahomogeneous, then $\mathrm{Aut}(A)$ contains such subgroup. Moreover, we characterize whose countable ultrahomogeneous posets $A$ such that for each natural $m$, the set of all cyclically dense elements $\bar{g}\in \mathrm{Aut}(A{)}^m$ for the diagonal action is comeager in $\mathrm{Aut}(A{)}^m$. In our considerations we strongly use the result of Schmerl which says that there are essentially four types of countably infinite ultrahomogeneous posets.