On disjoint crossing families in geometric graphs

TL;DR

This paper investigates the maximum number of edges in geometric graphs avoiding certain crossing configurations, establishing bounds and confirming a conjecture for specific cases, with implications for circle graphs.

Contribution

It proves an upper bound of c_k n log n edges for graphs without (k,k)-crossing families and confirms the conjecture for (2,1)-crossing families.

Findings

Graphs without (k,k)-crossing families have at most c_k n log n edges.

Confirmed the conjecture for graphs with no (2,1)-crossing family.

Derived bounds for geometric graphs avoiding specific intersection patterns.

Abstract

A geometric graph is a graph drawn in the plane with vertices represented by points and edges as straight-line segments. A geometric graph contains a (k,l)-crossing family if there is a pair of edge subsets E_1,E_2 such that |E_1| = k and |E_2| = l, the edges in E_1 are pairwise crossing, the edges in E_2 are pairwise crossing, and every edges in E_1 is disjoint to every edge in E_2. We conjecture that for any fixed k,l, every n-vertex geometric graph with no (k,l)-crossing family has at most c_{k,l}n edges, where c_{k,l} is a constant that depends only on k and l. In this note, we show that every n-vertex geometric graph with no (k,k)-crossing family has at most c_kn\log n edges, where c_k is a constant that depends only on k, by proving a more general result which relates extremal function of a geometric graph F with extremal function of two completely disjoint copies of F. We also settle the conjecture for geometric graphs with no (2,1)-crossing family. As a direct application, this implies that for any circle graph F on 3 vertices, every n-vertex geometric graph that does not contain a matching whose intersection graph is F has at most O(n) edges.