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November 28, 2008
Friday, November 28
Algebra Seminar
Time: 14:30
Speaker: Lex Renner (Western)
Title: "The H-polynomial of a group embedding"
Room: MC 107

Abstract: The Poincaré polynomial of a Weyl group calculates the Betti numbers of the projective homogeneous space G/B, while the h-vector of a simple polytope calculates the Betti numbers of the corresponding rationally smooth toric variety. There is a common generalization of these two extremes called the H-polynomial. It applies to projective, homogeneous spaces, toric varieties and, much more generally, to any algebraic variety X where there is a connected, solvable, algebraic group acting with a finite number of orbits. We illustrate this situation by describing the H-polynomials of certain projective (G x G)-varieties X, where G is a semisimple group and B is a Borel subgroup of G. This description is made possible by finding an appropriate cellular decomposition for X and then describing the cells combinatorially in terms of the underlying monoid of (B x B)-orbits. The most familiar example here is the wonderful compactification of a semisimple group of adjoint type.