On graphic arrangement groups

Abstract

A finite simple graph $\Gamma$ determines a quotient $P_\Gamma$ of the pure braid group, called a graphic arrangement group. We analyze homomorphisms of these groups defined by deletion of sets of vertices, using methods developed in prior joint work with R. Randell. We show that, for a $K_4$-free graph $\Gamma$, a product of deletion maps is injective, embedding $P_\Gamma$ in a product of free groups. Then $P_\Gamma$ is residually free, torsion-free, residually torsion-free nilpotent, and acts properly on a CAT(0) cube complex. We also show $P_\Gamma$ is of homological finiteness type $F_{m-1}$, but not $F_m$, where $m$ is the number of copies of $K_3$ in $\Gamma$, except in trivial cases. The embedding result is extended to graphs whose 4-cliques share at most one edge, giving an injection of $P_\Gamma$ into the product of pure braid groups corresponding to maximal cliques of $\Gamma$. We give examples showing that this map may inject in more general circumstances. We define the graphic braid group $B_\Gamma$ as a natural extension of $P_\Gamma$ by the automorphism group of $\Gamma$, and extend our homological finiteness result to these groups.