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

TL;DR

This paper investigates the decidability of closure and density questions for finitely generated subgroups of free groups under various pro-$k$-solvable topologies, providing effective methods and connections to pseudovarieties.

Contribution

It establishes decidability results for subgroup closures in pro-(met)abelian and pro-$V$ topologies, including effective construction of bases and membership decision procedures.

Findings

Decidability of subgroup closure and density in pro-(met)abelian topologies.

Effective construction of bases for closures when finitely generated.

Decidability of closure in pro-$V$ topologies for pseudovarieties like ${f S}_k$.

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-${\bf V}$ topology when ${\bf V}$ is an equational pseudovariety of finite groups, such as the pseudovariety ${\bf S}_k$ of all finite solvable groups with derived length $\leq k$. We also connect the pro-abelian topology with the topologies defined by abelian groups of bounded exponent.