Canonical representatives for divisor classes on tropical curves and the Matrix-Tree Theorem

Abstract

Let $\hat{I}“$ be a compact tropical curve (or metric graph) of genus $g$. Using the theory of tropical theta functions, Mikhalkin and Zharkov proved that there is a canonical effective representative (called a break divisor) for each linear equivalence class of divisors of degree $g$ on $\hat{I}“$. We present a new combinatorial proof of the fact that there is a unique break divisor in each equivalence class, establishing in the process an "integral" version of this result which is of independent interest. As an application, we provide a "geometric proof" of (a dual version of) Kirchhoff's celebrated Matrix-Tree Theorem. Indeed, we show that each weighted graph model $G$ for $\hat{I}“$ gives rise to a canonical polyhedral decomposition of the $g$-dimensional real torus ${\mathrm{Pic}}^g(\hat{I}“)$ into parallelotopes $C_T$, one for each spanning tree $T$ of $G$, and the dual Kirchhoff theorem becomes the statement that the volume of ${\mathrm{Pic}}^g(\hat{I}“)$ is the sum of the volumes of the cells in the decomposition.

20 pages -- Final version to appear in Forum of Math, Sigma