Homotopy coherent mapping class group actions and excision for Hochschild complexes of modular categories

Abstract

Given any modular category C over an algebraically closed field k, we extract a sequence (Mg)g≥0 of C-bimodules and show that the Hochschild chain complex CH(C;Mg) of C with coefficients in Mg carries a canonical homotopy coherent projective action of the mapping class group of the surface of genus g+1. The ordinary Hochschild complex of C corresponds to CH(C;M0).This result is obtained as part of the following more comprehensive topological structure: We construct a symmetric monoidal functor FC:C-Surfc⟶Chk with values in chain complexes over k defined on a symmetric monoidal category of surfaces whose boundary components are labeled with projective objects in C. The functor FC satisfies an excision property which is formulated in terms of homotopy coends. In this sense, any modular category gives naturally rise to a modular functor with values in chain complexes. In zeroth homology, it recovers Lyubashenko's mapping class group representations.The chain complexes in our construction are explicitly computable by choosing a marking on the surface, i.e. a cut system and a certain embedded graph. For our proof, we replace the connected and simply connected groupoid of cut systems that appears in the Lego-Teichmüller game by a contractible Kan complex.