Universal geometric coefficients for the once-punctured torus

Abstract

We construct universal geometric coefficients, over the integers, the rationals, and the reals, for cluster algebras arising from the once-punctured torus. We verify that the once-punctured torus has a property called the Null Tangle Property. The universal geometric coefficients over the integers and the rationals are then given by the shear coordinates of certain "allowable" curves in the torus. The universal geometric coefficients over the reals are given by the shear coordinates of allowable curves together with the normalized shear coordinates of certain other curves each of which is dense in the torus. We also construct the mutation fan for the once-punctured torus and recover a result of N\'ajera on g-vectors.