A proof of Feigin’s conjecture

Abstract

The paper is devoted to the proof of the following conjecture due to B. Feigin. Let $\frak u_\ell$ be the small quantum group a the primitive $\ell$-th root of unity. Then it is known that the usual $Ext$ algebra of the trivial $\frak u_\ell$-module is isomorphic to the algebra of regular functions on the nilpotent variety $\cal N$ in the corresponding simple Lie algebra $\frak g$ (see [GK]). Consider semiinfinite cohomology of the trivial $\frak u_\ell$-module introduced in [Ar1], [Ar2]. It was shown in [Ar2] that the $Ext$ algebra of the trivial $\frak u_\ell$-module acts naturally on semiinfinite cohomology. Moreover semiinfinite cohomology space of the trivial $\frak_\ell$-module is equipped with a natural $\frak g$-module structure. B. Feigin conjectured that the described $\frak g$-module and $F(\cal N)$-module structures coincide with the ones on the space of local cohomology of the structure sheaf on $\cal N$ with support in the standard positive nilpotent subalgebra $\frak n^+\subset\cal N\subset \frak g$. We give a detailed proof of the conjecture. Moreover we generalize the statement and describe semiinfinite cohomology of contragradient Weyl modules in terms of local cohomology of certain coherent sheaves on the nilpotent cone and on its desingularization $T^*(G/B)$. To do this we construct a certain specialization of the quantum BGG resolution of the simple module $L(\lambda)$ defined for generic values of the quantizing parameter into the root of unity called the quasi-BGG complex. The complex conists of direct sums of quasi-Verma modules and provides conjecturally a resolution of the Weyl module $W(\lambda)$.