Hedetniemi's Conjecture Via Altermatic Number

A $50$ years unsolved conjecture by Hedetniemi [{\it Homomorphisms of graphs and automata, \newblock {\em Thesis (Ph.D.)--University of Michigan}, 1966}] asserts that the chromatic number of the categorical product of two graphs $G$ and $H$ is $\min\{\chi(G),\chi(H)\}$. The present authors [{\it On the chromatic number of general {K}neser hypergraphs. \newblock {\em Journal of Combinatorial Theory, Series B}, 2015.}] introduced the altermatic and the strong altermatic number of graphs as two tight lower bounds for the chromatic number of graphs. In this work, we prove a relaxation of Hedetniemi's conjecture in terms of strong altermatic number. Also, we present a tight lower bound for the chromatic number of the categorical product of two graphs in term of their altermatic and strong altermatic numbers. These results enrich the family of pair graphs $\{G,H\}$ satisfying Hedetniemi's conjecture.