# Transfer-Matrix Methods meet Ehrhart Theory

## TL;DR

This paper explores the intersection of transfer-matrix methods and Ehrhart theory to jointly enumerate combinatorial structures with two parameters, revealing symmetries and geometric interactions.

## Contribution

It introduces a novel approach to jointly enumerate structures in transfer-matrix and Ehrhart frameworks, extending classical methods to handle two free parameters.

## Key findings

- Symmetry exploitation reduces computational complexity.
- Joint enumeration reveals new polynomial and quasi-polynomial behaviors.
- Connections between graph colorings and lattice point enumeration are established.

## Abstract

Transfer-Matrix Methods originated in physics where they were used to count the number of allowed particle states on a structure whose width $n$ is a parameter. Typically, the number of states is exponential in $n.$ One more mathematical instance of this methodology is to enumerate the proper vertex colorings of a graph of growing size by a fixed number of colors. In Ehrhart theory, lattice points in the dilation of a fixed polytope by a factor $k$ are enumerated. By inclusion-exclusion, relevant conditions on how the lattice points interact with hyperplanes are enforced. Typically, the number of points are (quasi-) polynomial in $k.$ The text-book example is that for a fixed graph, the number of proper vertex colorings with $k$ colors is polynomial in $k.$ This paper investigates the joint enumeration problem with both parameters $n$ and $k$ free. We start off with the classical graph colorings and then explore the common situations in combinatorics related to Ehrhart theory. We show how symmetries can be explored to reduce calculations and explain the interactions with Discrete Geometry.