Maximum diameter of $3$- and $4$-colorable graphs

TL;DR

This paper determines the maximum diameter of connected 3- and 4-colorable graphs with given order and minimum degree, solving a conjecture for these cases using linear programming duality.

Contribution

It provides a unified solution for the maximum diameter of 3- and 4-colorable graphs, extending previous conjectures and simplifying earlier results.

Findings

Maximum diameter bounds for 3- and 4-colorable graphs established

Unified linear programming approach applied to solve the problem

Simplification of previous complex results for 4-colorable graphs

Abstract

P. Erd\H{o}s, J. Pach, R. Pollack, and Z. Tuza [J. Combin. Theory, B 47 (1989), 279--285] made conjectures for the maximum diameter of connected graphs without a complete subgraph $K_{k+1}$, which have order $n$ and minimum degree $\delta$. Settling a weaker version of a problem, by strengthening the $K_{k+1}$-free condition to $k$-colorable, we solve the problem for $k=3$ and $k=4$ using a unified linear programming duality approach. The case $k=4$ is a substantial simplification of the result of \'E. Czabarka, P. Dankelmann, and L. A. Sz\'ekely [Europ. J. Comb., 30 (2009), 1082--1089].