A note on multicolour Ramsey numbers and random sphere graphs

Abstract

The Ramsey number $r(t;\ell )$ is the smallest $n$ such that every $\ell$-coloring of the edges of $K_n$ gives a monochromatic $K_t$. In recent years, there have been several improvements on asymptotic lower bounds for these numbers when $\ell \ge 3$. This started with a breakthrough result of Conlon and Ferber, followed by further improvements of Wigderson and then Sawin. Building on the previous approaches, Sawin used blowups of an unbalanced binomial random graph to show that there is some explicit constant $\hat{I}{´}_{\ast }\approx 0.383796$ such that $r(t;\ell )\ge 2^{\hat{I}{´}_{\ast }(\ell -2)t+t/2+o(t)}$. In this short note, we show that one can get an exponential improvement in this bound by replacing the use of a binomial random graph with a random sphere graph, a model which which has recently been applied by Ma, Shen and Xie in a breakthrough on lower bounds for (2-colour) Ramsey numbers in the (slightly) off-diagonal setting.

4 pages. The result presented here was obtained independently by Campos and Pohoata arXiv:2601.15183 and also appears in the Master's thesis of the second author