A generalization of crossing families

TL;DR

This paper introduces the concept of spoke sets as a generalization of crossing families in point sets, establishing a lower bound on their size and characterizing certain matchings derived from them.

Contribution

It defines spoke sets, proves that every point set has a spoke set of size at least rac{rac{n}{8}}, and characterizes specific matchings from unbounded regions.

Findings

Every point set has a spoke set of size rac{rac{n}{8}}.

Characterization of matchings connecting points in unbounded regions.

Extension of crossing family concepts to line arrangements in point sets.

Abstract

For a set of points in the plane, a \emph{crossing family} is a set of line segments, each joining two of the points, such that any two line segments cross. We investigate the following generalization of crossing families: a \emph{spoke set} is a set of lines drawn through a point set such that each unbounded region of the induced line arrangement contains at least one point of the point set. We show that every point set has a spoke set of size $\sqrt{\frac{n}{8}}$. We also characterize the matchings obtained by selecting exactly one point in each unbounded region and connecting every such point to the point in the antipodal unbounded region.