Six variations on a theme: almost planar graphs

Abstract

A graph is apex if it can be made planar by deleting a vertex, that is, $\exists v$ such that $G-v$ is planar. We define the related notions of edge apex, $\exists e$ such that $G-e$ is planar, and contraction apex, $\exists e$ such that $G/e$ is planar, as well as the analogues with a universal quantifier: $\forall v$, $G-v$ planar; $\forall e$, $G-e$ planar; and $\forall e$, $G/e$ planar. The Graph Minor Theorem of Robertson and Seymour ensures that each of these six gives rise to a finite set of obstruction graphs. For the three definitions with universal quantifiers we determine this set. For the remaining properties, apex, edge apex, and contraction apex, we show there are at least 36, 55, and 82 obstruction graphs respectively. We give two similar approaches to almost nonplanar ($\exists e$, $G+e$ is nonplanar and $\forall e$, $G+e$ is nonplanar) and determine the corresponding minor minimal graphs.