On Optimal 2- and 3-Planar Graphs

TL;DR

This paper characterizes the structure of optimal 2- and 3-planar graphs, revealing their simple regular form and exploring their relationships with other graph classes, advancing understanding of graphs with limited edge crossings.

Contribution

It provides a complete characterization of optimal 2- and 3-planar graphs, showing they have a simple regular structure and clarifying their inclusion relationships with other graph classes.

Findings

Optimal 2- and 3-planar graphs have a simple regular structure.

Characterization applies to non-simple graphs without homotopic parallel edges and self-loops.

New insights into inclusion relationships with other graph classes.

Abstract

A graph is $k$-planar if it can be drawn in the plane such that no edge is crossed more than $k$ times. While for $k=1$, optimal $1$-planar graphs, i.e., those with $n$ vertices and exactly $4n-8$ edges, have been completely characterized, this has not been the case for $k \geq 2$. For $k=2,3$ and $4$, upper bounds on the edge density have been developed for the case of simple graphs by Pach and T\'oth, Pach et al. and Ackerman, which have been used to improve the well-known "Crossing Lemma". Recently, we proved that these bounds also apply to non-simple $2$- and $3$-planar graphs without homotopic parallel edges and self-loops. In this paper, we completely characterize optimal $2$- and $3$-planar graphs, i.e., those that achieve the aforementioned upper bounds. We prove that they have a remarkably simple regular structure, although they might be non-simple. The new characterization allows us to develop notable insights concerning new inclusion relationships with other graph classes.