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Register a new userThe H-polynomial of a Group Embedding
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Authors
Lex E. Renner
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The Poincare 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 $Gtimes G$-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 $Btimes B$-orbits. The most familiar example here is the wonderful compactification of a semisimple group of adjoint type.
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