Let $G=G_1 \ast \ldots \ast G_k \ast F_N$ be a free product of finitely presented groups, where $F_N$ is a free group of rank $N \in \mathbb{ N}$. Let $\mathrm{ Out}(G,\mathcal{ G})$ be the subgroup of $\mathrm{ Out}(G)$ preserving the set of conjugacy classes $\mathcal{ G}=\{ [G_1],\ldots,[G_k]\}$. Under natural conditions on the groups $G_i$ with $i \in \{ 1,\ldots,k\}$, we prove that the group $\mathrm{ Out}(G,\mathcal{ G})$ has a finite index subgroup $\mathrm{ IA}(G,\mathcal{ G},3)$ with notable aperiodicity properties. We show that the group $\mathrm{ IA}(G,\mathcal{ G},3)$ is torsion free and, if $\phi \in \mathrm{ IA}(G,\mathcal{ G},3)$, every $\phi$-periodic conjugacy class of elements of $G$ is in fact fixed by $\phi$ and every $\phi$-periodic conjugacy class of free factors of $G$ is fixed by $\phi$. As an application, we prove that, for every toral relatively hyperbolic group $G$, the group $\mathrm{ Out}(G)$ has a finite index subgroup $\mathrm{ IA}(G,3)$ with the same above mentioned aperiodicity properties. We in particular give another proof of the theorem, due to Handel-Mosher, that the kernel of the action of $\mathrm{ Out}(F_N)$ on $H_1(F_N,\mathbb{ Z}/3\mathbb{ Z})$ satisfies natural aperiodicity properties.
Categories: math.GR math.GT