Desingularizations of the moduli space of rank 2 bundles over a curve

Abstract

Let X be a smooth projective curve of genus g≥3 and M0 be the moduli space of rank 2 semistable bundles over X with trivial determinant. There are three desingularizations of this singular moduli space constructed by Narasimhan-Ramanan [NR78], Seshadri [Ses77] and Kirwan [Kir86b] respectively. The relationship between them has not been understood so far. The purpose of this paper is to show that there is a morphism from Kirwan’s desingularization to Seshadri’s, which turns out to be the composition of two blow-downs. In doing so, we will show that the singularities of M0 are terminal and the plurigenera are all trivial. As an application, we compute the Betti numbers of the cohomology of Seshadri’s desingularization in all degrees. This generalizes the result of [BS90] which computes the Betti numbers in low degrees. Another application is the computation of the stringy E-function (see [Bat98] for definition) of M0 for any genus g≥3 which generalizes the result of [Kie03].