Overview on the theory of double flag varieties for symmetric pairs ...

Let $ G $ be a connected reductive algebraic group and its symmetric subgroup $ K $. The variety $ \dblFV = K/Q \times G/P $ is called a double flag variety, where $ Q $ and $ P $ are parabolic subgroups of $ K $ and $ G $ respectively. In this article, we make a survey on the theory of double flag varieties for a symmetric pair $ (G, K) $ and report entirely new results and theorems on this theory. Most important topic is the finiteness of $ K $-orbits on $ \dblFV $. We summarize the classification of $ \dblFV $ of finite type, which are scattered in the literatures. In some respects such classifications are complete, and in some cases not. In particular, we get a classification of double flag varieties of finite type when a symmetric pair is of type AIII, using the theorems of Homma who describes ``indecomposable'' objects of such double flag varieties. Together with these classifications, newly developed embedding theory provides double flag varieties of finite type, which are new. Other ingredients in ... : 70 pages ...