Automorphisms of profinite mapping class groups (2011.15075v4)

Abstract: For $S=S_{g,n}$ a closed orientable differentiable surface of genus $g$ from which $n$ points have been removed, such that $\chi(S)=2-2g-n<0$, let $\mathrm{P}\Gamma(S)$ be the pure mapping class group of $S$ and $\mathrm{P}\widehat\Gamma(S)$ and $\mathrm{P}\check\Gamma(S)$ be, respectively, its profinite and its congruence completions. The latter can be identified with the image of the natural representation $\mathrm{P}\widehat\Gamma(S)\to\operatorname{Out}({\widehat\pi}_1(S))$, where ${\widehat\pi}_1(S)$ is the profinite completion of the fundamental group of the surface $S$. Let $\operatorname{Out}^{\mathbb{I}_0}(\mathrm{P}\widehat\Gamma(S))$ and $\operatorname{Out}^{\mathbb{I}_0}(\mathrm{P}\check\Gamma(S))$ be the groups of outer automorphisms which preserve the conjugacy class of a procyclic subgroup generated by a nonseparating Dehn twist and let $\widehat{\operatorname{GT}}$ be the profinite Grothendieck-Teichm\"uller group. We then prove that, for $\chi(S)<g-2$, there is a natural faithful representation: [\widehat{\operatorname{GT}}\hookrightarrow\operatorname{Out}^{\mathbb{I}_0}(\mathrm{P}\widehat\Gamma(S))] and, letting $\Sigma_n$ be the symmetric group on the $n$ punctures of $S$, a natural isomorphism: [\operatorname{Out}^{\mathbb{I}_0}(\mathrm{P}\check\Gamma(S))\cong\Sigma_n\times\widehat{\operatorname{GT}}.]