Minors in graphs of large ${\theta}_r$-girth

TL;DR

This paper extends known results about the presence of clique-minors in graphs with large girth to graphs with large ${ heta}_r$-girth, replacing minimum degree with connectivity measures and analyzing minor exclusions.

Contribution

It generalizes the relationship between girth and clique-minors to ${ heta}_r$-girth, introducing new bounds involving connectivity and minor exclusion.

Findings

Graphs with large ${ heta}_r$-girth contain large clique-minors.

Connectivity can replace minimum degree in these bounds.

Graphs excluding certain minors have bounded treewidth.

Abstract

For every $r \in \mathbb{N}$, let $\theta_r$ denote the graph with two vertices and $r$ parallel edges. The $\theta_r$-girth of a graph $G$ is the minimum number of edges of a subgraph of $G$ that can be contracted to $\theta_r$. This notion generalizes the usual concept of girth which corresponds to the case $r=2$. In [Minors in graphs of large girth, Random Structures & Algorithms, 22(2):213--225, 2003], K\"uhn and Osthus showed that graphs of sufficiently large minimum degree contain clique-minors whose order is an exponential function of their girth. We extend this result for the case of $\theta_{r}$-girth and we show that the minimum degree can be replaced by some connectivity measurement. As an application of our results, we prove that, for every fixed $r$, graphs excluding as a minor the disjoint union of $k$ $\theta_{r}$'s have treewidth $O(k\cdot \log k)$.