# Enumerating linear systems on graphs

## TL;DR

This paper explores the enumeration of linear systems on graphs using divisor theory, connecting combinatorial, algebraic, and geometric perspectives, and providing explicit formulas and bijections for special cases.

## Contribution

It introduces methods to compute generating functions for all complete linear systems on graphs and relates these to polyhedral and invariant-theoretic structures, extending to M-matrix models.

## Key findings

- Generated explicit formulas for sizes of linear systems on graphs
- Established a bijection between linear systems and binary necklaces for cycle graphs
- Extended results to models based on integral M-matrices

## Abstract

The divisor theory of graphs views a finite connected graph $G$ as a discrete version of a Riemann surface. Divisors on $G$ are formal integral combinations of the vertices of $G$, and linear equivalence of divisors is determined by the discrete Laplacian operator for $G$. As in the case of Riemann surfaces, we are interested in the complete linear system $|D|$ of a divisor $D$---the collection of nonnegative divisors linearly equivalent to $D$. Unlike the case of Riemann surfaces, the complete linear system of a divisor on a graph is always finite. We compute generating functions encoding the sizes of all complete linear systems on $G$ and interpret our results in terms of polyhedra associated with divisors and in terms of the invariant theory of the (dual of the) Jacobian group of $G$. If $G$ is a cycle graph, our results lead to a bijection between complete linear systems and binary necklaces. The final section generalizes our results to a model based on integral $M$-matrices.