Combinatorics of Tripartite Boundary Connections for Trees and Dimers

TL;DR

This paper develops Pfaffian formulas for connection probabilities in groves and double-dimer models on planar graphs, enabling exact reconstruction of conductances from boundary data.

Contribution

It introduces Pfaffian formulas for tripartite boundary connections in groves and extends these results to the double-dimer model, generalizing previous determinant formulas.

Findings

Pfaffian formulas for tripartite connection probabilities

Exact reconstruction of conductances from boundary measurements

Extension of formulas to double-dimer models

Abstract

A grove is a spanning forest of a planar graph in which every component tree contains at least one of a special subset of vertices on the outer face called nodes. For the natural probability measure on groves, we compute various connection probabilities for the nodes in a random grove. In particular, for "tripartite" pairings of the nodes, the probability can be computed as a Pfaffian in the entries of the Dirichlet-to-Neumann matrix (discrete Hilbert transform) of the graph. These formulas generalize the determinant formulas given by Curtis, Ingerman, and Morrow, and by Fomin, for parallel pairings. These Pfaffian formulas are used to give exact expressions for reconstruction: reconstructing the conductances of a planar graph from boundary measurements. We prove similar theorems for the double-dimer model on bipartite planar graphs.