A sharp threshold phenomenon in string graphs

Abstract

We prove that for every $\epsilon>0$ there exists $\delta>0$ such that the following holds. Let $\mathcal{C}$ be a collection of $n$ curves in the plane such that there are at most $(\frac{1}{4}-\epsilon)\frac{n^{2}}{2}$ pairs of curves $\{\alpha,\beta\}$ in $\mathcal{C}$ having a nonempty intersection. Then $\mathcal{C}$ contains two disjoint subsets $\mathcal{A}$ and $\mathcal{B}$ such that $|\mathcal{A}|=|\mathcal{B}|\geq \delta n$, and every $\alpha\in \mathcal{A}$ is disjoint from every $\beta\in\mathcal{B}$. On the other hand, for every positive integer $n$ there exists a collection $\mathcal{C}$ of $n$ curves in the plane such that there at most $(\frac{1}{4}+\epsilon)\frac{n^{2}}{2}$ pairs of curves $\{\alpha,\beta\}$ having a nonempty intersection, but if $\mathcal{A},\mathcal{B}\subset \mathcal{C}$ are such that $|\mathcal{A}|=|\mathcal{B}|$ and $\alpha\cap \beta=\emptyset$ for every $(\alpha,\beta)\in \mathcal{A}\times\mathcal{B}$, then $|\mathcal{A}|=|\mathcal{B}|=O(\frac{1}{\epsilon}\log n)$.