From motivic Chern classes of Schubert cells to their Hirzebruch and CSM classes

Abstract

The equivariant motivic Chern class of a Schubert cell in a complete flag manifold X = G/B is an element in the equivariant K-theory ring of X to which one adjoins a formal parameter y. In this paper we prove several folklore results about motivic Chern classes, including finding specializations at y = −1 and y = 0; the coefficient of the top power of y; how to obtain Chern-Schwartz-MacPherson (CSM) classes as leading terms of motivic classes; divisibility properties of the Schubert expansion of motivic Chern classes. We collect several conjectures on the positivity, unimodality, and log concavity of CSM and motivic Chern classes of Schubert cells, including a conjectural positivity of structure constants of the multiplication of Poincar´e duals of CSM classes. In addition, we prove a ‘star duality’ for the motivic Chern classes, showing how they behave under the involution taking a vector bundle to its dual. We use the motivic Chern transformation to define two equivariant variants of the Hirzebruch transformation, which appear naturally in the Grothendieck-Hirzebruch-Riemann-Roch formalism. We utilize the Demazure-Lusztig recursions from the motivic Chern class theory to find similar recursions giving the Hirzebruch classes of Schubert cells, their Poincar´e duals, and their Segre versions. We explain the functoriality properties needed to extend the results to partial flag manifolds G/P.