Further Extensions of the Gr\"{o}tzsch Theorem

TL;DR

This paper extends the Gr"{o}tzsch Theorem to planar graphs with at most one triangle, providing new conditions under which partial 3-colorings can be extended to full colorings, and confirms a conjecture on adynamic coloring.

Contribution

It introduces new extension results for 3-colorings in planar graphs with limited triangles, including precoloring extensions and a proof of a conjecture on adynamic coloring.

Findings

Precoloring of two non-adjacent vertices extends to a 3-coloring.

Precoloring of a face of length at most 4 extends to a 3-coloring.

Precoloring neighborhood of a degree ≤3 vertex with the same color extends to a 3-coloring.

Abstract

The Gr\"{o}tzsch Theorem states that every triangle-free planar graph admits a proper $3$-coloring. Among many of its generalizations, the one of Gr\"{u}nbaum and Aksenov, giving $3$-colorability of planar graphs with at most three triangles, is perhaps the most known. A lot of attention was also given to extending $3$-colorings of subgraphs to the whole graph. In this paper, we consider $3$-colorings of planar graphs with at most one triangle. Particularly, we show that precoloring of any two non-adjacent vertices and precoloring of a face of length at most $4$ can be extended to a $3$-coloring of the graph. Additionally, we show that for every vertex of degree at most $3$, a precoloring of its neighborhood with the same color extends to a $3$-coloring of the graph. The latter result implies an affirmative answer to a conjecture on adynamic coloring. All the presented results are tight.