Paper detail

Strong accessibility of Coxeter groups over minimal splittings

Given a class of groups C, a group G is strongly accessible over C if there is a bound on the number of terms in a sequence L(1), L(2), ..., L(n) of graph of groups decompositions of G with edge groups in C such that L(1) is the trivial decomposition (with 1-vertex) and for i>1, L(i) is obtained from L(i-1) by non-trivially and compatibly splitting a vertex group of L(i-1) over a group in C, replacing this vertex group by the splitting and then reducing. If H and K are subgroups of a group G then H is smaller than K if H intersect K has finite index in H and infinite index in K. The minimal splitting subgroups of G, are the subgroups H of G, such that G splits non-trivially (as an amalgamated product or HNN-extension) over H and for any other splitting subgroup K of W, K is not smaller than H. When G is a finitely generated Coxeter group, minimal splitting subgroups are always finitely generated. Minimal splittings are explicitly or implicitly important aspects of Dunwoody's work on accessibility and the JSJ results of Rips-Sela, Dunwoody-Sageev and Mihalik. Our main results are that Coxeter groups are strongly accessible over minimal splittings and if L is an irreducible graph of groups decomposition of a Coxeter group with minimal splitting edge groups, then the vertex and edge groups of L are Coxeter.