Dual Garside structures and Coxeter sortable elements

Abstract

In Artin–Tits groups attached to spherical Coxeter groups, we give a combinatorial formula to express the simple elements of the dual braidmonoids in the classicalArtin generators. Every simple dual braid is obtained by lifting an $S$-reduced expression of its image in the Coxeter group, in a way which involves Reading’s $c$-sortable elements. It has as an immediate consequence that simple dual braids are Mikado braids (the known proofs of this result either require topological realizations of the Artin groups or categorification techniques), and hence that their images in the Iwahori–Hecke algebras have positivity properties. In the classical types, this requires to give an explicit description of the inverse of Reading’s bijection from noncrossing partitions of the Coxeter element $c$ to $c$-sortable elements, which might be of independent interest. The bijections are described in terms of the noncrossing partition models in these types. While the proof of the formula is case-by-case, it is entirely combinatorial and we develop an approach which reduces a uniform proof to uniformly proving a lemma about inversion sets of $c$-sortable elements.