Quasiplanar graphs, string graphs, and the Erdős–Gallai problem

Fox, J. and Pach, János and Suk, A. (2024) Quasiplanar graphs, string graphs, and the Erdős–Gallai problem. EUROPEAN JOURNAL OF COMBINATORICS. ISSN 0195-6698

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Abstract

An r-quasiplanar graph is a graph drawn in the plane with no r pairwise crossing edges. Let s ≥ 3 be an integer and r = 2s. We prove that there is a constant C such that every r-quasiplanar graph with n ≥ r vertices has at most n (Cs−1 log n)2s−4 edges. A graph whose vertices are continuous curves in the plane, two being connected by an edge if and only if they intersect, is called a string graph. We show that for every ϵ > 0, there exists δ > 0 such that every string graph with n vertices whose chromatic number is at least nϵ contains a clique of size at least nδ . A clique of this size or a coloring using fewer than nϵ colors can be found by a polynomial time algorithm in terms of the size of the geometric representation of the set of strings. In the process, we use, generalize, and strengthen previous results of Lee, Tomon, and others. All of our theorems are related to geometric variants of the following classical graph-theoretic problem of Erdős, Gallai, and Rogers. Given a Kr -free graph on n vertices and an integer s < r, at least how many vertices can we find such that the subgraph induced by them is Ks-free?

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