Quantized rational chip-firing

TL;DR

This paper introduces a quantized chip-firing model linked to rational lattice paths and parking functions, establishing combinatorial and algebraic structures with novel rounding rules and bijections.

Contribution

It develops a new quantized chip-firing framework with connections to rational combinatorics, defining stability notions and a group structure, and relating configurations to lattice paths and parking functions.

Findings

Superstable configurations correspond to rational parking functions.

A bijection exists between superstable and k-skeletal configurations.

The configuration set forms a group isomorphic to a product of cyclic groups.

Abstract

This article introduces a quantized chip-firing model with close connections to the theory of rational lattice paths and rational parking functions. Given a graph with a sink and positive integers a,b,c with gcd(a,b)=1, a set S of vertices fires by the following rule. Each vertex in S provisionally sends c chips to the sink and a/b chips to each non-sink neighbor outside of S. The novel feature is that the total number of chips leaving from or arriving at any vertex gets rounded down to the nearest integer before being finalized. We define the notions of chip configurations being superstable, k-stable, or k-skeletal in this model. When c=1 and the graph is complete, superstable configurations correspond to rational parking functions. There is a bijection between superstable configurations and k-skeletal configurations for each k. We establish these results by building a combinatorial theory of k-skeletal rational lattice paths (both unlabeled and labeled) and translating that theory to chip configurations. There is a group structure on the set of chip configurations modulo firing and borrowing moves. We show that this group is isomorphic to the product of b-1 copies of the integers modulo a; and, for each k, each coset of chip configurations in this group contains a unique k-skeletal representative.