Action of Hecke algebra on the double flag variety of type AIII

Abstract: Consider a connected reductive algebraic group $ G $ and a symmetric subgroup $ K $. Let $ \mathfrak{X} = K/B_K \times G/P $ be a double flag variety of finite type, where $ B_K $ is a Borel subgroup of $ K $, and $ P $ a parabolic subgroup of $ G $. A general argument shows that the orbit space $ \mathbb{C}\,\mathfrak{X}/K $ inherits a natural action of the Hecke algebra $ \mathscr{H} = \mathscr{H}(K, B_K) $ of double cosets via convolutions. However, to find out the explicit structure of the Hecke module is a quite different problem. In this paper, we determine the explicit action of $ \mathscr{H} $ on $ \mathbb{C}\,\mathfrak{X}/K $ in a combinatorial way using graphs for the double flag variety of type AIII. As a by-product, we also get the description of the representation of the Weyl group on $ \mathbb{C}\,\mathfrak{X}/K $ as a direct sum of induced representations.