Functions on Nilpotent Orbit Covers and Birational Geometry

Abstract

We use an analogue of the Springer resolution to describe the $G$-module structure on the ring of regular functions on the universal cover $\tilde{\mathcal{O}}$ of any nilpotent orbit for $G=S{L}_n$. Building on previous work on the extended Springer resolution, we construct a variety $\tilde{\mathcal{M}}$ that is finite over the cotangent bundle of a partial flag variety $G/P$, and proper and birational over the affinization $\mathcal{M}$ of $\tilde{\mathcal{O}}$. We use techniques in birational geometry to show that $\tilde{\mathcal{M}}$ has rational singularities, which provides the cohomology vanishing needed to describe the ring of functions on $\tilde{\mathcal{O}}$ as an induced representation from a Levi subgroup of $G$. Our results also yield a description of the structure of $R(\tilde{\mathcal{O}})$ as a graded $G$-module. We describe the minimal embedding of $\mathcal{M}$, study the lifting of characters of the component group of $\tilde{\mathcal{O}}$ to parabolics and Levi subgroups, and make a more general vanishing conjecture.