# Retracts of free groups and a question of Bergman

## TL;DR

This paper investigates the properties of retracts in free groups, proving specific cases where intersections are retracts and providing counterexamples, while also supporting the inertia conjecture for small subgroups.

## Contribution

It establishes when intersections of subgroups and retracts are themselves retracts in free groups, answering a question of Bergman and providing evidence for the inertia conjecture.

## Key findings

- For rank 2 subgroups, intersections with retracts are retracts.
- Counterexamples exist for higher ranks where intersections are not retracts.
- Supports the inertia conjecture for subgroups of rank up to 3.

## Abstract

Let $F_n$ be a free group of finite rank $n \geq 2$. We prove that if $H$ is a subgroup of $F_n$ with $\textrm{rk}(H)=2$ and $R$ is a retract of $F_n$, then $H \cap R$ is a retract of $H$. However, for every $m \geq 3$ and every $1 \leq k \leq n-1$, 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 $\textrm{rk}(H \cap \textrm{Fix}(S)) \leq \textrm{rk}(H)$ for every family $S$ of endomorphisms of $F_n$ and every subgroup $H$ of $F_n$ with $\textrm{rk}(H) \leq 3$.