The winding invariant

Abstract

Every element $w$ in the commutator subgroup of the free group $\mathbb{F}_2$ of rank 2 determines a closed curve in the grid $\mathbb{Z} \times \mathbb{R} \cup \mathbb{R} \times \mathbb{Z} \subseteq \mathbb{R}^2$. The winding numbers of this curve around the centers of the squares in the grid are the coefficients of a Laurent polynomial $P_w$ in two variables. This basic definition is related to well-known ideas in combinatorial group theory. We use this invariant to study equations over $\mathbb{F}_2$ and over the free metabelian group of rank $2$. We give a number of applications of algebraic, geometric and combinatorial flavor.