A framework unifying some bijections for graphs and its connection to Lawrence polytopes

TL;DR

This paper introduces a new family of bijections linking spanning trees and cycle-cocycle classes in graphs, based on Lawrence polytope triangulations, extending known bijections and applicable to regular matroids.

Contribution

It constructs a unified framework for bijections between spanning trees and orientations using Lawrence polytope triangulations, generalizing BBY and Bernardi bijections.

Findings

New bijections include BBY and Bernardi as special cases.

Bijections extend to subgraph-orientation correspondences.

Results hold for regular matroids.

Abstract

Let $G$ be a connected graph. The Jacobian group (also known as the Picard group or sandpile group) of $G$ is a finite abelian group whose cardinality equals the number of spanning trees of $G$. The Jacobian group admits a canonical simply transitive action on the set $\mathcal{R}(G)$ of cycle-cocycle reversal classes of orientations of $G$. Hence one can construct combinatorial bijections between spanning trees of $G$ and $\mathcal{R}(G)$ to build connections between spanning trees and the Jacobian group. The BBY bijections and the Bernardi bijections are two important examples. In this paper, we construct a new family of such bijections that includes both. Our bijections depend on a pair of atlases (different from the ones in manifold theory) that abstract and generalize certain common features of the two known bijections. The definitions of these atlases are derived from triangulations and dissections of the Lawrence polytopes associated to $G$. The acyclic cycle signatures and cocycle signatures used to define the BBY bijections correspond to regular triangulations. Our bijections can extend to subgraph-orientation correspondences. Most of our results hold for regular matroids. We present our work in the language of fourientations, which are a generalization of orientations.