SB-Labelings, Distributivity, and Bruhat Order on Sortable Elements

In this article, we investigate the set of $γ$-sortable elements, associated with a Coxeter group $W$ and a Coxeter element $γ\in W$, under Bruhat order, and we denote this poset by $\mathcal{B}_γ$. We show that this poset belongs to the class of SB-lattices recently introduced by Hersh and Mészáros, by proving a more general statement, namely that all join-distributive lattices are SB-lattices. The observation that $\mathcal{B}_γ$ is join-distributive is due to Armstrong. Subsequently, we investigate for which finite Coxeter groups $W$ and which Coxeter elements $γ\in W$ the lattice $\mathcal{B}_γ$ is in fact distributive. It turns out that this is the case for the "coincidental" Coxeter groups, namely the groups $A_{n},B_{n},H_{3}$ and $I_{2}(k)$. We conclude this article with a conjectural characteriziation of the Coxeter elements $γ$ of said groups for which $\mathcal{B}_γ$ is distributive in terms of forbidden orientations of the Coxeter diagram.

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