On the rational cohomology of spin hyperelliptic mapping class groups

Abstract

Let $\mathfrak{G}$ be the subgroup $\mathfrak{S}_{n-q} \times \mathfrak{S}_{q}$ of the $n$-th symmetric group $\mathfrak{S}_{n}$ for $n-q \geq q$. In this paper, we study the $\mathfrak{G}$-invariant part of the rational cohomology group of the pure braid group $P_{n}$. The invariant part includes the rational cohomology of a spin hyperelliptic mapping class group of genus $g$ as a subalgebra when $n=2g+2$, denoted by $H^*(P_{n})^{\mathfrak{G}}$. Based on the study of Lehrer-Solomon, we prove that they are independent of $n$ and $q$ in degree $*\leq q-1$. We also give a formula to calculate the dimension of $H^*(P_{n})^{\mathfrak{G}}$ and calculate it in all degree for $q\leq 3$.