A Crossing Lemma for Jordan curves

If two Jordan curves in the plane have precisely one point in common, and there they do not properly cross, then the common point is called a touching point. The main result of this paper is a Crossing Lemma for simple curves: Let X and T stand for the sets of intersection points and touching points...

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Bibliographic Details
Published in:Advances in mathematics (New York. 1965) 2018-06, Vol.331, p.908-940
Main Authors: Pach, János, Rubin, Natan, Tardos, Gábor
Format: Article
Language:English
Subjects:
Arrangements of curves
Combinatorial geometry
Contact graphs
Crossing Lemma
Extremal problems
Separators
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Summary:If two Jordan curves in the plane have precisely one point in common, and there they do not properly cross, then the common point is called a touching point. The main result of this paper is a Crossing Lemma for simple curves: Let X and T stand for the sets of intersection points and touching points, respectively, in a family of n simple curves in the plane, no three of which pass through the same point. If |T|>cn, for some fixed constant c>0, then we prove that |X|=Ω(|T|(log⁡log⁡(|T|/n))1/504). In particular, if |T|/n→∞, then the number of intersection points is much larger than the number of touching points. As a corollary, we confirm the following long-standing conjecture of Richter and Thomassen: The total number of intersection points between n pairwise intersecting simple closed (i.e., Jordan) curves in the plane, no three of which pass through the same point, is at least (1−o(1))n2.
ISSN:0001-8708
1090-2082
DOI:10.1016/j.aim.2018.03.015