Closed polylines with fixed self-intersection index

TL;DR

This paper explores the existence of closed polylines with fixed self-intersection counts, providing complete solutions for certain intersection numbers and general existence results for large polylines.

Contribution

It offers a complete characterization for polylines with 3, 4, and 6 self-intersections and proves existence conditions for large polylines with fixed intersection counts.

Findings

Complete solutions for k=3,4,6 cases

Non-existence theorems for certain configurations

Existence of polylines for large n when nk is even

Abstract

We investigate the existence of closed polylines (also known as closed polygonal chains or self-crossing polygons) that intersect each of their edges the same number of times. The most general question in this corner of combinatorial geometry asks for all pairs $(n, k)$ such that there exists a closed polyline with $n$ edges, each intersecting the same polyline exactly $k$ times. For $k = 1$ and $k = 2$, this is a very simple question answered several decades ago. In this article, we present a complete solution for $k = 3, 4, 6$, as well as the proof of some non-existence theorems. In conclusion, we show that, for an arbitrary positive integer $k$, a polyline of the required type exists for any sufficiently large integer $n$ such that $nk$ is even.