Reducing quadrangulations of the sphere and the projective plane

TL;DR

This paper demonstrates how quadrangulations of the sphere and projective plane can be simplified through specific graph operations, revealing conditions for t-perfection related to bipartiteness.

Contribution

It introduces a method to reduce quadrangulations to basic graphs using t-contractions and deletions, linking t-perfection to bipartiteness in the projective plane.

Findings

Sphere quadrangulations reduce to 4-cycles

Projective plane quadrangulations reduce to odd wheels

t-perfection characterized by bipartiteness

Abstract

We show that every quadrangulation of the sphere can be transformed into a $4$-cycle by deletions of degree-$2$ vertices and by $t$-contractions at degree-$3$ vertices. A $t$-contraction simultaneously contracts all incident edges at a vertex with stable neighbourhood. The operation is mainly used in the field of $t$-perfect graphs. We further show that a non-bipartite quadrangulation of the projective plane can be transformed into an odd wheel by $t$-contractions and deletions of degree-$2$ vertices. We deduce that a quadrangulation of the projective plane is (strongly) $t$-perfect if and only if the graph is bipartite.