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# Leonardo Mihalcea (Virginia Tech), Geometric Methods in Representation Theory Seminar

## September 22, 2017 @ 4:00 pm - 5:00 pm

**Title: ***Chern-Schwartz-MacPherson classes for Schubert cells: geometry and representation theory *

**Abstract: **A compact manifold has a tangent bundle, and a natural question is to find a replacement for the Chern classes of the tangent

bundle, in the case when the space is singular. The Chern-Schwartz-MacPherson (CSM) classes are homology classes which “behave like” the Chern classes of the tangent bundle, and are determined by a functoriality property. The existence of these classes was conjectured by Grothendieck and Deligne, and proved by MacPherson in 1970’s. The calculation of the CSM classes for Schubert cells and Schubert varieties in flag manifolds was obtained only recently, and it

exhibited some unexpected features. For instance, these classes are determined by a Demazure-Lusztig operator, and they are essentially equivalent to certain Lagrangian cycles in the cotangent bundle of the flag manifold, showing up in the proof of Kazhdan-Lusztig conjectures. They are also equivalent to the stable envelopes of Maulik and Okounkov. In this talk I will survey some of these developments. No prior knowledge about the CSM classes will be assumed. This is joint work with P. Aluffi, and ongoing joint work with P. Aluffi, J. Schurmann and C. Su.