A geometric model for blocks of Frobenius kernels

Abstract

Building on a geometric counterpart of Steinberg's tensor product formula for simple representations of a connected reductive algebraic group q G over a field of positive characteristic proved in [AR3], and following an idea of Arkhipov-Bezrukavnikov-Braverman-Gaitsgory-Mirković, we define and initiate the study of some categories of perverse sheaves on the affine Grassmannian of the Langlands dual group to q G that should provide geometric models for blocks of representations of the Frobenius kernel q G 1 of q G. In particular, we show that these categories admit enough projective and injective objects, which are closely related to some tilting perverse sheaves, and that they are highest weight categories in an appropriate generalized sense. Contents 1. Introduction 1 2. Combinatorics of the affine Weyl group 6 3. Perverse sheaves on affine Grassmannians 13 4. Background from representation theory 26 5. Modules over the regular perverse sheaf 33 6. Averaging and wall-crossing functors 43 7. Projectives and injectives 51 8. Ungraded R-modules 60 9. Baby Verma and co-Verma modules 67 References 87