The Erd\H{o}s-P\'osa property for infinite graphs

Abstract

We investigate which classes of infinite graphs have the Erd\H{o}s-P\'osa property (EPP). In addition to the usual EPP, we also consider the following infinite variant of the EPP: a class $\mathcal{G}$ of graphs has the $\kappa$-EPP, where $\kappa$ is an infinite cardinal, if for any graph $\Gamma$ there are either $\kappa$ disjoint graphs from $\mathcal{G}$ in $\Gamma$ or there is a set $X$ of vertices of $\Gamma$ of size less than $\kappa$ such that $\Gamma - X$ contains no graph from $\mathcal{G}$. In particular, we study the ($\kappa$-)EPP for classes consisting of a single infinite graph $G$. We obtain positive results when the set of induced subgraphs of $G$ is labelled well-quasi-ordered, and negative results when $G$ is not a proper subgraph of itself (both results require some additional conditions). As a corollary, we obtain that every graph which does not contain a path of length $n$ for some $n \in \mathbb{N}$ has the EPP and the $\kappa$-EPP. Furthermore, we show that the class of all subdivisions of any tree $T$ has the $\kappa$-EPP for every uncountable cardinal $\kappa$, and if $T$ is rayless, also the $\aleph_0$-EPP and the EPP.