Half-integral Erdős-Pósa property for non-null $S$-$T$ paths

Abstract

For a group $Γ$, a $Γ$-labelled graph is an undirected graph $G$ where every orientation of an edge is assigned an element of $Γ$ so that opposite orientations of the same edge are assigned inverse elements. A path in $G$ is non-null if the product of the labels along the path is not the neutral element of $Γ$. We prove that for every finite group $Γ$, non-null $S$-$T$ paths in $Γ$-labelled graphs exhibit the half-integral Erdős-Pósa property. More precisely, there is a function $f$, depending on $Γ$, such that for every $Γ$-labelled graph $G$, subsets of vertices $S$ and $T$, and integer $k$, one of the following objects exists: a family $\cal F$ consisting of $k$ non-null $S$-$T$ paths in $G$ such that every vertex of $G$ participates in at most two paths of $\cal F$; or a set $X$ consisting of at most $f(k)$ vertices that meets every non-null $S$-$T$ path in $G$. This in particular proves that in undirected graphs $S$-$T$ paths of odd length have the half-integral Erdős-Pósa property.