On the Homotopy Type of Intersections of Two Real Bruhat Cells

Abstract

Real Bruhat cells give an important and well-studied stratification of such spaces as $\operatorname {GL}_{n+1}$, $\operatorname {Flag}_{n+1} = \operatorname {SL}_{n+1}/B$, $\operatorname {SO}_{n+1}$, and $\operatorname {Spin}_{n+1}$. We study the intersections of a top dimensional cell with another cell (for another basis). Such an intersection is naturally identified with a subset of the lower nilpotent group $\operatorname {Lo}_{n+1}^{1}$. We are particularly interested in the homotopy type of such intersections. In this paper, we define a stratification of such intersections. As a consequence, we obtain a finite CW complex that is homotopically equivalent to the intersection. We compute the homotopy type for several examples. It turns out that for $n \le 4$ all connected components of such subsets of $\operatorname {Lo}_{n+1}^1$ are contractible: we prove this by explicitly constructing the corresponding CW complexes. Conversely, for $n \ge 5$ and the top permutation, there is always a connected component with even Euler characteristic and therefore not contractible. This follows from formulas for the number of cells per dimension of the corresponding CW complex. For instance, for the top permutation $S_6$, there exists a connected component with Euler characteristic equal to $2$. We also give an example of a permutation in $S_6$ for which there exists a connected component that is homotopically equivalent to the circle ${{\mathbb {S}}}^1$.