The Springer correspondence makes a link between the characters of a Weyl group and the geometry of the nilpotent cone of the corresponding semisimple Lie algebra. In this article, we consider a modular version of the theory, and show that the decomposition numbers of a Weyl group are particular cases of decomposition numbers for equivariant perverse sheaves on the nilpotent cone. We give some decomposition numbers which can be obtained geometrically. In the case of the symmetric group, we show that James' row and column removal rule for the symmetric group can be derived from a smooth equivalence between nilpotent singularities proved by Kraft and Procesi. We give the complete structure of the Springer and Grothendieck sheaves in the case of @math . Finally, we determine explicitly the modular Springer correspondence for exceptional types.
We should also mention @cite_12 which, apart from being a continuation of @cite_14 , contains many compatibilities, notably of induction and restriction functors with respect to a modular Springer functor defined in terms of restriction to the nilpotent cone. By @cite_15 , those compatibilities are valid also for the Fourier transform construction of the present paper.
@cite_15 @cite_14 @cite_12
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Given the nilpotent cone of a complex reductive Lie algebra, we consider its equivariant constructible derived category of sheaves with coefficients in an arbitrary field. This category and its subcategory of perverse sheaves play an important role in Springer theory and the theory of character sheaves. We show that the composition of the Fourier--Sato transform on the Lie algebra followed by restriction to the nilpotent cone restricts to an autoequivalence of the derived category of the nilpotent cone. In the case of @math , we show that this autoequivalence can be regarded as a geometric version of Ringel duality for the Schur algebra. We show that two Weyl group actions on the Springer sheaf with arbitrary coefficients, one defined by Fourier transform and one by restriction, agree up to a twist by the sign character. This generalizes a familiar result from the setting of l-adic cohomology, making it applicable to modular representation theory. We use the Weyl group actions to define a Springer correspondence in this generality, and identify the zero weight spaces of small representations in terms of this Springer correspondence. We show that the topological modular functor from Witten-Chern-Simons theory is universal for quantum computation in the sense that a quantum circuit com- putation can be efficiently approximated by an intertwining action of a braid on the functor's state space. A computational model based on Chern-Simons theory at a fifth root of unity is defined and shown to be polynomially equivalent to the quantum circuit model. The chief technical advance: the density of the irreducible sectors of the Jones representation has topological implications which will be considered elsewhere. We give geometric descriptions of the category C_k(n,d) of rational polynomial representations of GL_n over a field k of degree d for d less than or equal to n, the Schur functor and Schur-Weyl duality. The descriptions and proofs use a modular version of Springer theory and relationships between the equivariant geometry of the affine Grassmannian and the nilpotent cone for the general linear groups. Motivated by this description, we propose generalizations for an arbitrary connected complex reductive group of the category C_k(n,d) and the Schur functor.