An improved lower bound for multicolor Ramsey numbers and the half-multiplicity Ramsey number problem

Abstract

The multicolor Ramsey number problem asks, for each pair of natural numbers $\ell$ and $t$, for the largest $\ell$-coloring of a complete graph with no monochromatic clique of size $t$. Recent works of Conlon-Ferber and Wigderson have improved the longstanding lower bound for this problem. We make a further improvement by replacing an explicit graph appearing in their constructions by a random graph. Graphs useful for this construction are exactly those relevant for a problem of Erd\H{o}s on graphs with no large cliques and few large independent sets. We also make some basic observations about this problem.