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

Abstract

P. Erdős, 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 É. Czabarka, P. Dankelmann, and L. A. Székely [Europ. J. Comb., 30 (2009), 1082--1089].

8 pages. arXiv admin note: text overlap with arXiv:2009.02611