Quickly excluding a non-planar graph

TL;DR

This paper presents a new, constructive, and polynomial-time method to quickly determine if a graph is non-planar by leveraging a simplified proof of a key structural theorem in graph theory, with explicit constants.

Contribution

It provides a shorter, constructive proof of a fundamental graph minor structure theorem using only prior results and textbooks, enabling polynomial-time algorithms.

Findings

The proof is shorter and more accessible than previous ones.

It yields explicit constants for the structure theorem.

The approach allows for polynomial-time algorithms to exclude non-planar graphs.

Abstract

A cornerstone theorem in the Graph Minors series of Robertson and Seymour is the result that every graph $G$ with no minor isomorphic to a fixed graph $H$ has a certain structure. The structure can then be exploited to deduce far-reaching consequences. The exact statement requires some explanation, but roughly it says that there exist integers $k,n$ depending on $H$ only such that $0<k<n$ and for every $n\times n$ grid minor $J$ of $G$ the graph $G$ has a a $k$-near embedding in a surface $\Sigma$ that does not embed $H$ in such a way that a substantial part of $J$ is embedded in $\Sigma$. Here a $k$-near embedding means that after deleting at most $k$ vertices the graph can be drawn in $\Sigma$ without crossings, except for local areas of non-planarity, where crossings are permitted, but at most $k$ of these areas are attached to the rest of the graph by four or more vertices and inside those the graph is constrained in a different way, again depending on the parameter $k$. The original and only proof so far is quite long and uses many results developed in the Graph Minors series. We give a proof that uses only our earlier paper [A new proof of the flat wall theorem, {\it J.~Combin.\ Theory Ser.\ B \bf 129} (2018), 158--203] and results from graduate textbooks. Our proof is constructive and yields a polynomial time algorithm to construct such a structure. We also give explicit constants for the structure theorem, whereas the original proof only guarantees the existence of such constants.