Normality of closures of conormal bundles to Schubert cells for $\operatorname{GL}_n$

Abstract

We prove normality of closures of the conormal bundle to certain Schubert cells in the flag variety for $\operatorname{GL}_n$ coming from Grassmannians. Our argument uses a Zelevinsky-type resolution of singularities, and through a sequence of cohomological computations based on the Borel--Weil--Bott theorem, we reduce normality to a problem about Kronecker coefficients, which we solve by applying results of Knutson and Tao from honeycomb theory.