On the intersection density of the Kneser Graph $K(n,3)$

Abstract

A set $\mathcal{F}\subset \mathrm{Sym}(V)$ is \textsl{intersecting} if any two of its elements agree on some element of $V$. Given a finite transitive permutation group $G\le \mathrm{Sym}(V)$, the \textsl{intersection density} $\rho (G)$ is the maximum ratio $\frac{|\mathcal{F}||V|}{|G|}$ where $\mathcal{F}$ runs through all intersecting sets of $G$. The \textsl{intersection density} $\rho (X)$ of a vertex-transitive graph $X=(V,E)$ is equal to $\max \left\{ ρ(G) : G \leq \operatorname{Aut}(X), \mbox{ $G$ transitive} \right\}$. In this paper, we study the intersection density of the Kneser graph $K(n,3)$, for $n\ge 7$. The intersection density of $K(n,3)$ is determined whenever its automorphism group contains ${\mathrm{PSL}}_2(q)$, with some exceptional cases depending on the congruence of $q$. We also briefly consider the intersection density of $K(n,2)$ for values of $n$ where ${\mathrm{PSL}}_2(q)$ is a subgroup of its automorphism group.

14 pages, final version, to appear in the European Journal of Combinatorics