Convex cocompact subgroups of mapping class groups.
Commensurations of Out
Let Out(F) denote the outer automorphism group of the free group F with >3. We prove that for any finite index subgroup Γ<Out(F), the group Aut(Γ) is isomorphic to the normalizer of Γ in Out(F). We prove that Γ is : every injective homomorphism Γ→Γ is surjective. Finally, we prove that the abstract commensurator Comm(Out(F)) is isomorphic to Out(F).
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