LOCALLY INVARIANT ORDERS ON GROUPS

Abstract

Recent work by T. Delzant and S. Hair shows that certain groups are unique product groups. In effect, they show that the groups have a locally invariant order, an idea introduced by D. Promislow in the early eighties. Having a locally invariant order implies the group is a unique product group, and a strict left (or right) ordering on a group is a locally invariant order. We study properties of the class of LIO groups, that is, groups having a locally invariant order. The main result gives conditions under which the fundamental group of a graph of LIO groups is LIO. In particular, the free product of two LIO groups is LIO. There is an analogous result for a graph of right orderable groups. We also study tree-free groups (those having a free action without inversions on a Λ-tree, for some ordered abelian group Λ). In particular, a detailed proof that tree-free groups are LIO is given. There is also a detailed proof of an observation made by Hair, that the fundamental group of a compact hyperbolic manifold is virtually LIO.