From Trees to Tripods: Proof of $K(\pi ,1)$ for Artin groups with $ABI$-type spherical parabolics

Abstract

We reduce the $K(\pi ,1)$-conjecture for all Artin groups with tree Coxeter diagrams to properties of Artin groups with tripod-shaped Coxeter diagrams. Combining this reduction theorem and properties of braid groups in previous works of Charney, Crisp-McCammond, Haettel and the second named author, we deduce that the $K(\pi ,1)$-conjecture holds for every Artin group whose spherical parabolic subgroups avoid type $D_n$ ($n\ge 4$) and the exceptional types. The reduction theorem relies on producing a ``tower'' of injective metric spaces from a single Artin group. The construction of such a tower relies on two ingredients of independent interests: a notion of combinatorial convexity and a Bestvina-type inequality, in certain injective orthoscheme complexes. These ingredients further rely on the use of structural properties of bi-Helly graphs (also known as absolute bipartite retracts) developed in joint work of the first named author with Munro.

53 pages, 9 figures