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On the Richter–Thomassen Conjecture about Pairwise Intersecting Closed Curves†

Part of: Extremal combinatorics Graph theory Discrete geometry

Published online by Cambridge University Press:  04 May 2016

JÁNOS PACH ,
NATAN RUBIN  and
GÁBOR TARDOS
JÁNOS PACH
Affiliation:
École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland and Alfréd Rényi Institute of Mathematics, Realtanoda utca 13-15, H-1053, Budapest (e-mail: pach@cims.nyu.edu)
NATAN RUBIN
Affiliation:
Department of Computer Science, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel (e-mail: rubinnat.ac@gmail.com)
GÁBOR TARDOS
Affiliation:
Alfréd Rényi Institute of Mathematics, Realtanoda utca 13-15, H-1053, Budapest (e-mail: tardos@renyi.hu)
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Abstract

A long-standing conjecture of Richter and Thomassen states that the total number of intersection points between any n simple closed Jordan curves in the plane, so that any pair of them intersect and no three curves pass through the same point, is at least (1−o(1))n 2.

We confirm the above conjecture in several important cases, including the case (1) when all curves are convex, and (2) when the family of curves can be partitioned into two equal classes such that each curve from the first class touches every curve from the second class. (Two closed or open curves are said to be touching if they have precisely one point in common and at this point the two curves do not properly cross.)

An important ingredient of our proofs is the following statement. Let S be a family of n open curves in ℝ2, so that each curve is the graph of a continuous real function defined on ℝ, and no three of them pass through the same point. If there are nt pairs of touching curves in S, then the number of crossing points is $\Omega(nt\sqrt{\log t/\log\log t})$ .

MSC classification

Primary: 52C45: Combinatorial complexity of geometric structures
Secondary: 05C10: Planar graphs; geometric and topological aspects of graph theory 05C35: Extremal problems 05D99: None of the above, but in this section 52C10: Erdős problems and related topics of discrete geometry 52C30: Planar arrangements of lines and pseudolines
Type
Paper
Information
Combinatorics, Probability and Computing , Volume 25 , Issue 6 , November 2016 , pp. 941 - 958
Copyright
Copyright © Cambridge University Press 2016 

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Footnotes

†

A preliminary version of this paper appeared in Proc. 26th Annual ACM–SIAM Symposium on Discrete Algorithms (2015), pp. 1506–1516.

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