2-extendability of (4,5,6)-fullerenes

TL;DR

This paper characterizes which (4,5,6)-fullerenes are 2-extendable, identifying specific sporadic cases and classes, and reveals a link between non-2-extendability and the anti-Kekulé number.

Contribution

It completely classifies 2-extendability of (4,5,6)-fullerenes, including sporadic and class-based cases, and establishes new connections with anti-Kekulé numbers.

Findings

All non-2-extendable (4,5,6)-fullerenes are identified.

All (4,5,6)-fullerenes with anti-Kekulé number 3 are non-2-extendable.

Existence of non-2-extendable (4,5,6)-fullerenes with arbitrarily many vertices.

Abstract

A (4,5,6)-fullerene is a plane cubic graph whose faces are only quadrilaterals, pentagons and hexagons, which includes all (4,6)- and (5,6)-fullerenes. A connected graph $G$ with at least $2k+2$ vertices is $k$-extendable if $G$ has perfect matchings and any matching of size $k$ is contained in a perfect matching of $G$. We know that each (4,5,6)-fullerene graph is 1-extendable and at most 2-extendable. It is natural to wonder which (4,5,6)-fullerene graphs are 2-extendable. In this paper, we completely solve this problem (see Theorem 3.3): All non-2-extendable (4,5,6)-fullerenes consist of four sporadic (4,5,6)-fullerenes ($F_{12},F_{14},F_{18}$ and $F_{20}$) and five classes of (4,5,6)-fullerenes. As a surprising consequence, we find that all (4,5,6)-fullerenes with the anti-Kekul\'{e} number 3 are non-2-extendable. Further, there also always exists a non-2-extendable (4,5,6)-fullerene with arbitrarily even $n\geqslant10$ vertices.