# Coloring count cones of planar graphs

## TL;DR

This paper studies the mathematical structure of coloring extension counts in planar graphs, introduces a conjecture related to the Four Color Theorem, and provides supporting evidence for it.

## Contribution

It characterizes the coloring count cone for certain planar graphs, formulates a stronger conjecture than the Four Color Theorem, and offers evidence supporting this conjecture.

## Key findings

- The coloring count function belongs to a specific cone in the function space.
- A new conjecture strengthening the Four Color Theorem is proposed.
- Evidence supporting the conjecture is presented.

## Abstract

For a plane near-triangulation $G$ with the outer face bounded by a cycle $C$, let $n^\star_G$ denote the function that to each $4$-coloring $\psi$ of $C$ assigns the number of ways $\psi$ extends to a $4$-coloring of $G$. The block-count reducibility argument (which has been developed in connection with attempted proofs of the Four Color Theorem) is equivalent to the statement that the function $n^\star_G$ belongs to a certain cone in the space of all functions from $4$-colorings of $C$ to real numbers. We investigate the properties of this cone for $|C|=5$, formulate a conjecture strengthening the Four Color Theorem, and present evidence supporting this conjecture.