A generalized Cartan decomposition for the double coset space U(n₁) x U(n₂) x U(n₃)) U(n) / U(p) x U(q)

Motivated by recent developments on visible actions on complex manifolds, we raise a question whether or not the multiplication of three subgroups $L$, $G'$ and $H$ surjects a Lie group $G$ in the setting that $G/H$ carries a complex structure and contains $G'/G' \cap H$ as a totally real submanifold. Particularly important cases are when $G/L$ and $G/H$ are generalized flag varieties, and we classify pairs of Levi subgroups $(L, H)$ such that $L G' H = G$, or equivalently, the real generalized flag variety $G'/H \cap G'$ meets every $L$-orbit on the complex generalized flag variety $G/H$ in the setting that $(G, G') = (U(n), O(n))$. For such pairs $(L, H)$, we introduce a \textit{herringbone stitch} method to find a generalized Cartan decomposition for the double coset space $L \backslash G/H$, for which there has been no general theory in the non-symmetric case. Our geometric results provides a unified proof of various multiplicity-free theorems in representation theory of general linear groups.