JSJ Decompositions of Coxeter Groups

Abstract

The idea of "JSJ-decompositions" for 3-manifolds began with work of Waldhausen and was developed later through work of Jaco, Shalen and Johansen. It was shown that there is a finite collection of 2-sided, incompressible tori that separate a given closed irreducible 3-manifold into pieces with strong topological structure. Sela introduced the idea of JSJ-decompositions for groups, an idea that has flourished in a variety of directions. The general idea is to consider a certain class X of groups and splittings of groups in X by groups in another class Y. E.g. Rips and Sela considered splittings of finitely presented groups by infinite cyclic groups. For an arbitrary group G in X the goal is to produce a unique graph of groups decomposition T of G with edge groups in Y so that T reveals all graph of groups decompositions of G with edge groups in Y. More specifically, if V is a vertex group of T then either there is no Y-group that splits both G and V, or V has a special "surface group-like" structure. It is standard to call vertex groups of the second type "orbifold groups". For a finitely generated Coxeter system (W,S) we produce a reduced JSJ-decomposition T for splittings of W over virtually abelian subgroups. We show T is unique with each vertex and edge group generated by a subset of S (and so T is "visual"). The construction of T is algorithmic. If V, a subset of S, generates an orbifold vertex group of T then V is the disjoint union of K and M, where < M > is virtually abelian, < K > is virtually a closed surface group or virtually free and < V > is the direct product of < M > and < K >.