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Cell decompositions of double Bott-Samelson varieties

Let G be a connected complex semisimple Lie group. Webster and Yakimov have constructed partitions of the double flag variety G/B x G/B_, where (B, B_) is a pair of opposite Borel subgroups of G, generalizing the Deodhar decompositions of G/B. We show that these partitions can be better understood by constructing cell decompositions of a product of two Bott-Samelson varieties Z_{u, v}, where u and v are sequences of simple reflections. We construct coordinates on each cell of the decompositions and in the case of a positive subexpression, we relate these coordinates to regular functions on a particular open subset of Z_{u, v}. Our motivation for constructing cell decompositions of Z_{u,v} was to study a certain natural Poisson structure on Z_{u,v}.