# Maximum Rectilinear Crossing Number of Uniform Hypergraphs

## TL;DR

This paper advances the understanding of rectilinear crossing numbers in high-dimensional hypergraphs, providing new bounds, confirming conjectures for 4D cases, and establishing NP-hardness for general hypergraphs.

## Contribution

It improves lower bounds, proves a conjecture for 4D hypergraphs, and shows NP-hardness of computing maximum rectilinear crossing numbers.

## Key findings

- Improved lower bound for $d$-dimensional rectilinear crossing number of complete hypergraphs.
- Confirmed the conjecture for maximum crossing pairs in 4D hypergraphs.
- Established NP-hardness for the problem in arbitrary hypergraphs.

## Abstract

We improve the lower bound on the $d$-dimensional rectilinear crossing number of the complete $d$-uniform hypergraph having $2d$ vertices to $\Omega\left(\dfrac{(4\sqrt{2}/3^{3/4})^d}{d}\right)$ from $\Omega(2^d \sqrt{d})$. We also establish that the $3$-dimensional rectilinear crossing number of a complete $3$-uniform hypergraph having $n \geq 9$ vertices is at least $\dfrac{43}{42}\dbinom{n}{6}$. We prove that the maximum number of crossing pairs of hyperedges in a $4$-dimensional rectilinear drawing of the complete $4$-uniform hypergraph having $n$ vertices is $13\dbinom{n}{8}$. We also prove that among all $4$-dimensional rectilinear drawings of a complete $4$-uniform hypergraph having $n$ vertices, the number of crossing pairs of hyperedges is maximized if all its vertices are placed at the vertices of a $4$-dimensional neighborly polytope. Our result proves the conjecture by Anshu et al. [Anshu, Gangopadhyay, Shannigrahi, and Vusirikala, 2017] for $d=4$. We prove that the maximum $d$-dimensional rectilinear crossing number of a complete $d$-partite $d$-uniform balanced hypergraph is $(2^{d-1}-1){\dbinom{n}{2}}^d$. We then prove that finding the maximum $d$-dimensional rectilinear crossing number of an arbitrary $d$-uniform hypergraph is NP-hard. We give a randomized scheme to create a $d$-dimensional rectilinear drawing of a $d$-uniform hypergraph $H$ such that, in expectation the total number of crossing pairs of hyperedges is a constant fraction of the maximum $d$-dimensional rectilinear crossing number of $H$.