On the Boxicity of Kneser Graphs and Complements of Line Graphs

Abstract

An axis-parallel $d$-dimensional box is a cartesian product $I_1\times I_2\times \dots \times I_b$ where $I_i$ is a closed sub-interval of the real line. For a graph $G = (V,E)$, the $boxicity \ of \ G$, denoted by $\text{box}(G)$, is the minimum dimension $d$ such that $G$ is the intersection graph of a family $(B_v)_{v\in V}$ of $d$-dimensional boxes in $\mathbb R^d$. Let $k$ and $n$ be two positive integers such that $n\geq 2k+1$. The $Kneser \ graph$ $Kn(k,n)$ is the graph with vertex set given by all subsets of $\{1,2,\dots,n\}$ of size $k$ where two vertices are adjacent if their corresponding $k$-sets are disjoint. In this note, we derive a general upper bound for $\text{box}(Kn(k,n))$, and a lower bound in the case $n\ge 2k^3-2k^2+1$, which matches the upper bound up to an additive factor of $\Theta(k^2)$. Our second contribution is to provide upper and lower bounds for the boxicity of the complement of the line graph of any graph $G$, and as a corollary, we derive that $\text{box}(Kn(2,n))\in \{n-3, n-2\}$ for every $n\ge 5$.