Separable elements and Bruhat orders

Source abstract

We prove the curious fact that, in any Coxeter group, the intersection of a principal left weak order ideal $[e,x]_L$ with a principal right weak order ideal $[e,y]_R$ has a unique maximal element $\mathrm{lr}(x,y)$ under Bruhat order. We apply this to resolve a conjecture of Hart and Rowley on involutions in weak order ideals. This fact arises from, and is applied to, our study of the theory of separable elements in finite Weyl groups. We use it to prove that for $U=[e,u]_R$ in a Coxeter group $W$, the multiplication map $W/U\times U\rightarrow W$ is surjective, where $W/U$ is the Björner-Wachs generalized quotient. This resolves and generalizes a conjecture of the authors. For finite Weyl groups, we further prove that this map is bijective if and only if $u$ is separable, thereby classifying the splittings of finite Weyl groups into weak order intervals, and resolving the natural extension of a question of Björner-Wachs.