Multi-cones over Schubert varieties (Q1820839)

The authors prove the Cohen-Macaulayness of multicones over Schubert varieties. Let G be a reductive algebraic group, B a Borel subgroup and X a Schubert variety in G/B. Let \(L_ i\), \(1\leq i\leq n\) be the line bundles corresponding to the fundamental weights (here, \(n=rank(G))\). Let \(L=\otimes^{n}_{i=1}L_ i^{a_ i},\quad a_ i\in {\mathbb{Z}}^+\), \(1\leq i\leq n\). Let \(R=\oplus_{L}H^ 0(X,L)\quad\) and \(C=Spec(R)\). The authors prove that the ring C is Cohen-Macaulay. In the course of proving this, they also give a criterion for C to have rational singularities, if X does (similar to Serre's criterion for arithmetic Cohen-Macaulayness). The proof of the main theorem uses Frobenius splitting of Schubert varieties. The Frobenius splitting of Schubert varieties was first proved by Mehta and Ramanathan. Subsequently, Ramanathan has proved geometric properties for Schubert varieties like arithmetic normality, arithmetic Cohen-Macaulayness etc., using Frobenius splitting. The result about the Cohen-Macaulayness of multicones completes the picture regarding Cohen-Macaulay properties that arise in the context of Schubert varieties. The paper is a nice contribution to the geometric study of Schubert varieties.