Cycle Spaces of Infinite Dimensional Flag Domains

Abstract: Let $G$ be a complex simple direct limit group, specifically $SL(\infty;\mathbb{C})$, $SO(\infty;\mathbb{C})$ or $Sp(\infty;\mathbb{C})$. Let $\mathcal{F}$ be a (generalized) flag in $\mathbb{C}^\infty$. If $G$ is $SO(\infty;\mathbb{C})$ or $Sp(\infty;\mathbb{C})$ we suppose further that $\mathcal{F}$ is isotropic. Let $\mathcal{Z}$ denote the corresponding flag manifold; thus $\mathcal{Z} = G/Q$ where $Q$ is a parabolic subgroup of $G$. In a paper with Ignatyev and Penkov, we studied real forms $G_0$ of $G$ and properties of their orbits on $\mathcal{Z}$. Here we concentrate on open $G_0$--orbits $D \subset \mathcal{Z}$. When $G_0$ is of hermitian type we work out the complete $G_0$--orbit structure of flag manifolds dual to the bounded symmetric domain for $G_0$. Then we develop the structure of the corresponding cycle spaces $\mathcal{M}_D$. Finally we study the real and quaternionic analogs of these theories. All this extends an large body of results from the finite dimensional cases on the structure of hermitian symmetric spaces and related cycle spaces.