The Even Isomorphism Theorem for Coxeter Groups

Coxeter groups have presentations 〈S : (st)st∀s, t ∈ S〉 where for all s, t ∈ S, mst ∈ {1, 2, . . . ,∞}, mst = mts and mst = 1 if and only if s = t. A fundamental question in the theory of Coxeter groups is: Given two such “Coxeter” presentations, do they present the same group? There are two known ways to change a Coxeter presentation, generally referred to as twisting and simplex exchange. We solve the isomorphism question for Coxeter groups with an even Coxeter presentation (one in which mst is even or ∞ when s 6= t). More specifically, we give an algorithm that describes a sequence of twists and triangle-edge exchanges that either converts an arbitrary finitely generated Coxeter presentation into a unique even presentation or identifies the group as a non-even Coxeter group. Our technique can be used to produce all Coxeter presentations for a given even