On the local-global conjecture for commutator traces

Abstract

We study the trace set of the commutator subgroup of Γ(2), a type of Local-Global problem about thin groups. We determine the local obstructions and then use the correspondence between binary quadratic forms and hyperbolic matrices to find some global obstructions. We then develop a probabilistic argument for the existence of sufficiently large admissible traces by modeling the elements as non-backtracking random walks in a 2-dimensional lattice via their homology class and word length. Finally, we investigate the number of commutators needed to represent matrices in the commutator subgroup. This is done using an algorithm of Goldstein and Turner along with utilizing properties of level k-Markoff type surfaces. We conjecture that any trace in the commutator subgroup of Γ(2) can be represented with either 1 or 2 commutators.