On the Richter-Thomassen Conjecture about Pairwise Intersecting Closed Curves

TL;DR

This paper proves the Richter-Thomassen conjecture for certain cases, showing that pairwise intersecting simple closed curves in the plane have at least roughly n^2 intersection points, with new results for convex and bipartite touching configurations.

Contribution

The paper confirms the conjecture in cases where all curves are convex or form a bipartite touching family, providing new bounds on intersection points.

Findings

Confirmed the conjecture for convex curves.

Established bounds for touching and crossing points in bipartite families.

Provided a lower bound on crossings based on touching pairs.

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 is touching every curve from the second class. (Two curves are said to be touching if they have precisely one point in common, at which they do not properly cross.) An important ingredient of our proofs is the following statement: Let $S$ be a family of the graphs of $n$ continuous real functions defined on $\mathbb{R}$, no three of which pass through the same point. If there are $nt$ pairs of touching curves in $S$, then the number of crossing points is $\Omega(nt\sqrt{\log t/\log\log t})$.