Homotopy Type of Intersections of Real Bruhat Cells in Dimension 6

TL;DR

This paper explores the homotopy types of intersections of real Bruhat cells in 6x6 matrices, revealing most are contractible except for one case with a circle-like structure.

Contribution

It provides a detailed analysis of the homotopy types of intersections of real Bruhat cells in dimension 6, identifying non-contractible components and their topological nature.

Findings

Most intersections are contractible.

Identified a non-contractible component with circle homotopy type.

Extended understanding of Bruhat cell intersections in dimension 6.

Abstract

In this work, we investigate the arbitrary intersection of real Bruhat cells. Such objects have attracted interest from various authors, particularly due to their appearance in different contexts: such as in Kazhdan-Lusztig theory and in the study of locally convex curves. We study the homotopy type of the intersection of two real Bruhat cells. This homotopy type is the same as that of an explicit submanifold of the group of real lower triangular matrices with diagonal entries equal to 1. For $(n+1)\times(n+1)$ matrices with $n\leq4$, these submanifolds are the disjoint union of contractible connected components. Our focus is on such intersections for $6\times6$ real matrices. For this, we study the connected components of Bruhat cells for permutations $\sigma\in\Sn_6$ with at most 12 inversions. We make use of the structure of the dual CW complexes associated with these components. We show that for permutations with at most 12 inversions, with the exception of $\sigma=[563412]$ , all connected components are contractible. Furthermore, for $\sigma=[563412]$, we identify new non-contractible connected components with the homotopy type of the circle.