Unfriendly partitions when avoiding vertices of finite degree

Abstract

An unfriendly partition of a graph $G=(V,E)$ is a function $c:V\to 2$ such that $|\{x\in N(v):c(x)\ne c(v)\}|\ge |\{x\in N(v):c(x)=c(v)\}|$ for every vertex $v\in V$, where $N(v)$ denotes its neighborhood. It was conjectured by Cowen and Emerson that every graph has an unfriendly partition, but Milner and Shelah found counterexamples for that statement by analyzing graphs with uncountably many vertices. Curiously, none of their graphs have vertices with finite degree. Therefore, as a natural direction to approach, in this paper we search for the least cardinality of a graph with that property that admits no unfriendly partitions. Actually, among some other independence results, we conclude that this size cannot be determined from the usual axioms of set theory.

8 pages