On the cohomology of pure mapping class groups as FI-modules

Abstract

In this paper we apply the theory of finitely generated FI-modules developed by Church et al. to certain sequences of rational cohomology groups. An FI-module over \({\mathbb {Q}}\) is a functor from the category of finite sets and injections to the category of vector spaces over \({\mathbb {Q}}\). Our main examples are the cohomology of the moduli space of \(n\)-pointed curves, the cohomology of the pure mapping class group of surfaces and some manifolds of higher dimension, and the cohomology of classifying spaces of some diffeomorphism groups. We introduce the notion of FI\([G]\)-module and use it to strengthen and give new context to results on representation stability discussed by the author in a previous paper. Moreover, we prove that the Betti numbers of these spaces and groups are polynomial and find bounds on their degree. Finally, we obtain rational homological stability of certain wreath products.