Separation dimension of sparse graphs

Abstract

The separation dimension of a graph $G$ is the smallest natural number $k$ for which the vertices of $G$ can be embedded in $\mathbb{R}^k$ such that any pair of disjoint edges in $G$ can be separated by a hyperplane normal to one of the axes. Equivalently, it is the smallest possible cardinality of a family $\mathcal{F}$ of permutations of the vertices of $G$ such that for any two disjoint edges of $G$, there exists at least one permutation in $\mathcal{F}$ in which all the vertices in one edge precede those in the other. In general, the maximum separation dimension of a graph on $n$ vertices is $\Theta(\log n)$. In this article, we focus on sparse graphs and show that the maximum separation dimension of a $k$-degenerate graph on $n$ vertices is $O(k \log\log n)$ and that there exists a family of $2$-degenerate graphs with separation dimension $\Omega(\log\log n)$. We also show that the separation dimension of the graph $G^{1/2}$ obtained by subdividing once every edge of another graph $G$ is at most $(1 + o(1)) \log\log \chi(G)$ where $\chi(G)$ is the chromatic number of the original graph.