Fixed subgroups are compressed in surface groups

Abstract

For a compact surface $Σ$ (orientable or not, and with boundary or not) we show that the fixed subgroup, $\operatorname{Fix} B$, of any family $B$ of endomorphisms of $π_1(Σ)$ is compressed in $π_1(Σ)$ i.e., $\operatorname{rk}((\operatorname{Fix} B)\cap H)\leq \operatorname{rk}(H)$ for any subgroup $\operatorname{Fix} B \leq H \leq π_1(Σ)$. On the way, we give a partial positive solution to the inertia conjecture, both for free and for surface groups. We also investigate direct products, $G$, of finitely many free and surface groups, and give a characterization of when $G$ satisfies that $\operatorname{rk}(\operatorname{Fix} ϕ) \leq \operatorname{rk}(G)$ for every $ϕ\in Aut(G)$.