Integral Flow and Cycle Chip-Firing on Graphs

Abstract

Motivated by the notion of chip-firing on the dual graph of a planar graph, we consider ‘integral flow chip-firing’ on an arbitrary graph G. The chip-firing rule is governed by \({\mathcal {L}}^*(G)\), the dual Laplacian of G determined by choosing a basis for the lattice of integral flows on G. We show that any graph admits such a basis so that \({\mathcal {L}}^*(G)\) is an M-matrix, leading to a firing rule on these basis elements that is avalanche finite. This follows from a more general result on bases of integral lattices that may be of independent interest. Our results provide a notion of z-superstable flow configurations that are in bijection with the set of spanning trees of G. We show that for planar graphs, as well as for the graphs \(K_5\) and \(K_{3,3}\), one can find such a flow M-basis that consists of cycles of the underlying graph. We consider the question for arbitrary graphs and address some open questions.