Automorphisms of profinite mapping class groups

Abstract

For $S=S_{g,n}$ a closed orientable differentiable surface of genus $g$ from which $n$ points have been removed, such that $χ(S)=2-2g-n<0$, let $\mathrm{P}Γ(S)$ be the pure mapping class group of $S$ and $\mathrm{P}\widehatΓ(S)$ and $\mathrm{P}\checkΓ(S)$ be, respectively, its profinite and its congruence completions. The latter can be identified with the image of the natural representation $\mathrm{P}\widehatΓ(S)\to\operatorname{Out}({\widehatπ}_1(S))$, where ${\widehatπ}_1(S)$ is the profinite completion of the fundamental group of the surface $S$. Let $\operatorname{Out}^{\mathbb{I}_0}(\mathrm{P}\widehatΓ(S))$ and $\operatorname{Out}^{\mathbb{I}_0}(\mathrm{P}\checkΓ(S))$ 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-Teichmüller group. We then prove that, for $χ(S)<g-2$, there is a natural faithful representation: \[\widehat{\operatorname{GT}}\hookrightarrow\operatorname{Out}^{\mathbb{I}_0}(\mathrm{P}\widehatΓ(S))\] and, letting $Σ_n$ be the symmetric group on the $n$ punctures of $S$, a natural isomorphism: \[\operatorname{Out}^{\mathbb{I}_0}(\mathrm{P}\checkΓ(S))\congΣ_n\times\widehat{\operatorname{GT}}.\]

Theorems & Definitions (118)

• Theorem A
• Theorem B
• Theorem C
• Definition 2.1
• Definition 2.2
• Theorem 2.3
• Remark 2.4
• Definition 2.5
• Proposition 2.6
• proof