Retracts of free groups and a question of Bergman

Abstract

Let $F_n$ be a free group of finite rank $n\ge 2$. We prove that if $H$ is a subgroup of $F_n$ with $\text{rk}(H)=2$ and $R$ is a retract of $F_n$, then $H\cap R$ is a retract of $H$. However, for every $m\ge 3$ and every $1\le k\le n-1$, there exist a subgroup $H$ of $F_n$ of rank $m$ and a retract $R$ of $F_n$ of rank $k$ such that $H\cap R$ is not a retract of $H$. This gives a complete answer to a question of Bergman. Furthermore, we provide positive evidence for the inertia conjecture of Dicks and Ventura. More precisely, we prove that $\text{rk}(H\cap \text{Fix}(S))\le \text{rk}(H)$ for every family $S$ of endomorphisms of $F_n$ and every subgroup $H$ of $F_n$ with $\text{rk}(H)\le 3$.