The pro-$k$-solvable topology on a free group

Abstract

We prove that, given a finitely generated subgroup $H$ of a free group $F$, the following questions are decidable: is $H$ closed (dense) in $F$ for the pro-(met)abelian topology? is the closure of $H$ in $F$ for the pro-(met)abelian topology finitely generated? We show also that if the latter question has a positive answer, then we can effectively construct a basis for the closure, and the closure has decidable membership problem in any case. Moreover, it is decidable whether $H$ is closed for the pro-$\mathbf{V}$ topology when $\mathbf{V}$ is an equational pseudovariety of finite groups, such as the pseudovariety ${\mathbf{S}}_k$ of all finite solvable groups with derived length $\le k$. We also connect the pro-abelian topology with the topologies defined by abelian groups of bounded exponent.