Flag manifolds, spaces of frames, and connectedness properties

Abstract

For integers $1\le k<n$ and real numbers $c_1,\dots ,c_k>0$ and $d_1,\dots ,d_n\ge 0$, we investigate the space of all $k\times n$ matrices $F$ such that the product $F{F}^{\ast }$ is equal to the diagonal matrix $\mathrm{Diag}(c_1,\dots ,c_k)$ and the squared norms of the columns of $F$ are equal to $d_1,\dots ,d_n$ respectively. Depending on the field where the coefficients of $F$ are taken from, which can be of real or of complex numbers, we are mainly interested in determining whether the resulting space is path-connected or even simply connected relative to the subspace topology in the space of all $k\times n$ matrices. The criteria presented are closely related to results previously obtained by Cahill, Mixon, and Strawn (2017), Needham and Shonkwiler (2021), the last two authors together with Caine (2026), and the author of this work (2024 and 2026). Flag manifolds, namely orbits of the canonical conjugation actions of $\mathrm{O}(n)$ and $\mathrm{U}(n)$ on the spaces of symmetric real and Hermitian $n\times n$ matrices, respectively, play a central role in our development.

31 pages