On the cohomology ring of the moduli space of rank 2 vector bundles on a curve

Let $\MS_g$ be the moduli space of stable holomorphic vector bundles of rank 2 and fixed determinant of odd degree over a smooth complex projective curve of genus $g$. This paper proves various properties of the rational cohomology ring $H^*(\MS_g)$. It is shown that the first relation in genus $g$ between the standard generators satisfies a recurrence relation in $g$ and that the invariant subring for the mapping class group is a complete intersection ring. (These two results have been obtained independently by Zagier, Baranovsky and Siebert & Tian.) A Gr\"obner basis is found for the ideal of invariant relations. A structural formula for $H^*(\MS_g)$ (originally conjectured by Mumford) is verified and a natural monomial basis is given.