Ricerc@Sapienza

Let G be a connected reductive complex algebraic group and B a Borel subgroup of G. We consider a subgroup H of B which acts with finitely many orbits on the flag variety G / B, and we classify the H-orbits in G / B in terms of suitable root systems. As well, we study the Weyl group action defined by Knop on the set of H-orbits in G / B, and we give a combinatorial model for this action in terms of weight polytopes.