Associativity and Thompson's Group

Ross Geoghegan, Fernando Guzman

Published 2005-05-23Version 1

Given a set S equipped with a binary operation (we call this a "bracket algebra") one may ask to what extent the binary operation satisfies some of the consequences of the associative law even when it is not actually associative? We define a subgroup Assoc(S) of Thompson's Group F for each bracket algebra S, and we interpret the size of Assoc(S) as determining the amount of associativity in S - the larger Assoc(S) is, the more associativity holds in S. When S is actually associative, Assoc(S) = F; that is the trivial case. In general, it turns out that only certain subgroups of F can occur as Assoc(S) for some S, and we describe those subgroups precisely. We then explain what happens in some familiar examples: Lie algebras with the Lie bracket as binary operation, groups with the commutator bracket as binary operation, the Cayley numbers with their usual multiplication, as well as some less familiar examples. In the case of a group G, with the commutator bracket as binary operation, it is better to think of the "virtual size of G", determined by all the groups Assoc(H) such that H is a subgroup of finite index in G. This gives a way of partitioning groups into "small", "intermediate" and "large" - a partition suggestive of, but different from, traditional measures of a group's size such as growth, isoperimetric inequality and "amenable vs. non-amenable"