Transitive subtournaments of $k$-th power Paley digraphs and improved lower bounds for Ramsey numbers

Abstract

Let $k \geq 2$ be an even integer. Let $q$ be a prime power such that $q \equiv k+1 \pmod {2k}$. We define the $\textit{k-th power Paley digraph}$ of order $q$, $G_k(q)$, as the graph with vertex set $\mathbb{F}_q$ where $a \to b$ is an edge if and only if $b-a$ is a $k$-th power residue. This generalizes the (k=2) Paley Tournament. We provide a formula, in terms of finite field hypergeometric functions, for the number of transitive subtournaments of order four contained in $G_k(q)$, $\mathcal{K}_4(G_k(q))$, which holds for all $k$. We also provide a formula, in terms of Jacobi sums, for the number of transitive subtournaments of order three contained in $G_k(q)$, $\mathcal{K}_3(G_k(q))$. In both cases, we give explicit determinations of these formulae for small $k$. We show that zero values of $\mathcal{K}_4(G_k(q))$ (resp. $\mathcal{K}_3(G_k(q))$) yield lower bounds for the multicolor directed Ramsey numbers $R_{\frac{k}{2}}(4)=R(4,4,\cdots,4)$ (resp. $R_{\frac{k}{2}}(3)$). We state explicitly these lower bounds for $k\leq 10$ and compare to known bounds, showing improvement for $R_2(4)$ and $R_3(3)$. Combining with known multiplicative relations we give improved lower bounds for $R_{t}(4)$, for all $t\geq 2$, and for $R_{t}(3)$, for all $t \geq 3$.