On the transition monoid of the Stallings automaton of a subgroup of a free group

Abstract

Birget, Margolis, Meakin and Weil proved that a finitely generated subgroup $K$ of a free group is pure if and only if the transition monoid $M(K)$ of its Stallings automaton is aperiodic. In this paper, we establish further connections between algebraic properties of $K$ and algebraic properties of $M(K)$. We mainly focus on the cases where $M(K)$ belongs to the pseudovariety $\overline{\boldsymbol{\mathbf{{H}}}}$ of finite monoids all of whose subgroups lie in a given pseudovariety $\overline{\boldsymbol{\mathbf{{H}}}}$ of finite groups. We also discuss normal, malnormal and cyclonormal subgroups of $F_A$ using the transition monoid of the corresponding Stallings automaton.