A Class of Rank — 2 Torsion Free Groups

Abstract

This section has little to do with the main theme. I once thought that the notion to be discussed here was key to classifying finite rank torsion free groups; I still feel that it is the fundamental invariant for such groups. It seems to be in the back of the mind of averyone who studies these groups and it surfaces, more or less explicitly, in many discussions. I include it with the lope of giving it some formal standing.