Refute plane diamond coloring conjecture with 16-vertex counterexample

Adds Theorem 2.6 stating Conjecture 2.5 is false, with proof exhibiting
a 16-vertex maximal planar graph (graph6 O???IAGKEBEQQYHdplW{n) for
which the auxiliary 4-colorability check fails at every root vertex,
verified computationally via Sage's chromatic_number. Includes the
graph as a figure and adds a McKay graph6 reference.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
This commit is contained in:
2026-05-09 12:40:42 -04:00
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@@ -36,6 +36,9 @@
\documentclass{amsart}
\usepackage{graphicx}
\usepackage{url}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{proposition}[theorem]{Proposition}
@@ -147,6 +150,39 @@ The conjecture below asserts a structural property of $4$-colorings of maximal p
Every maximal planar graph $G$ has a plane diamond coloring.
\end{conjecture}
\begin{theorem}
Conjecture 2.5 is false.
\end{theorem}
\begin{proof}
Let $G$ be the maximal planar graph on $16$ vertices with graph6 string\footnote{We use the standard graph6 encoding of McKay; see \cite{mckaygraph6}.}
\[
\texttt{O???IAGKEBEQQYHdplW\{n}
\]
shown in Figure~\ref{fig:counterexample}. Equivalently, $G$ has edge set
\begin{align*}
&\{0,8\}, \{0,10\}, \{0,14\}, \{1,7\}, \{1,10\}, \{1,12\}, \{2,9\}, \{2,11\}, \{2,13\}, \\
&\{3,9\}, \{3,11\}, \{3,15\}, \{4,8\}, \{4,12\}, \{4,14\}, \{4,15\}, \{5,6\}, \{5,13\}, \\
&\{5,14\}, \{5,15\}, \{6,11\}, \{6,13\}, \{6,15\}, \{7,10\}, \{7,12\}, \{7,14\}, \{8,10\}, \\
&\{8,12\}, \{8,14\}, \{9,11\}, \{9,13\}, \{9,15\}, \{10,12\}, \{10,14\}, \{11,13\}, \\
&\{11,15\}, \{12,13\}, \{12,14\}, \{12,15\}, \{13,14\}, \{13,15\}, \{14,15\}.
\end{align*}
We have $|V(G)| = 16$ and $|E(G)| = 42 = 3 \cdot 16 - 6$, so $G$ is a triangulation.
By Proposition 2.4, it suffices to verify that for every choice of root $u \in V(G)$ and every pair of distinct colors $c_a, c_b$, no proper $4$-coloring $C$ of $G$ satisfies $C^{-1}(c_a) \subseteq \bigcup_{i \text{ even}} L_i^{(u)}$ and $C^{-1}(c_b) \subseteq \bigcup_{i \text{ odd}} L_i^{(u)}$, where $\{L_i^{(u)}\}$ denotes the distance partition from $u$.
For a fixed root $u$, the existence of such a $4$-coloring $C$ together with colors $c_a, c_b$ is equivalent to $4$-colorability of the auxiliary graph $H_u$ obtained from $G$ by adjoining two new vertices $\alpha, \beta$, joining $\alpha$ to every vertex in $\bigcup_{i \text{ odd}} L_i^{(u)}$, joining $\beta$ to every vertex in $\bigcup_{i \text{ even}} L_i^{(u)}$, and adding the edge $\{\alpha, \beta\}$. Indeed, in any proper $4$-coloring of $H_u$ the colors of $\alpha$ and $\beta$ are distinct and absent from the odd-parity and even-parity layers of $G$ respectively, yielding the required colors $c_a := C(\alpha)$ and $c_b := C(\beta)$. Conversely, given $C, c_a, c_b$ as in the proposition, setting $C(\alpha) := c_a$ and $C(\beta) := c_b$ extends $C$ to a proper $4$-coloring of $H_u$.
A direct computation (using \texttt{Sage}'s \texttt{chromatic\_number}) verifies that $\chi(H_u) > 4$ for every $u \in V(G)$, so $G$ admits no plane diamond coloring.
\end{proof}
\begin{figure}[htbp]
\centering
\includegraphics[width=0.45\textwidth]{counterexample.png}
\caption{A maximal planar graph on $16$ vertices with no plane diamond coloring.}
\label{fig:counterexample}
\end{figure}
\begin{thebibliography}{9}
\bibitem{appel1977every}
@@ -159,6 +195,11 @@ N.~Robertson, D.~Sanders, P.~Seymour, and R.~Thomas,
\emph{The four-colour theorem},
Journal of Combinatorial Theory, Series B, vol.~70, no.~1, pp.~2--44, 1997.
\bibitem{mckaygraph6}
B.~D.~McKay,
\emph{Description of graph6, sparse6 and digraph6 encodings},
\url{https://users.cecs.anu.edu.au/~bdm/data/formats.txt}.
\end{thebibliography}
\end{document}