Extremal functions for sparse minors
Abstract
The "extremal function" $c(H)$ of a graph $H$ is the supremum of densities of graphs not containing $H$ as a minor, where the "density" of a graph $G$ is the ratio of the number of edges to the number of vertices. Myers and Thomason (2005), Norin, Reed, Thomason and Wood (2020), and Thomason and Wales (2019) determined the asymptotic behaviour of $c(H)$ for all polynomially dense graphs $H$, as well as almost all graphs $H$ of constant density. We explore the asymptotic behavior of the extremal function in the regime not covered by the above results, where in addition to having constant density the graph $H$ is in a graph class admitting strongly sublinear separators. We establish asymptotically tight bounds in many cases. For example, we prove that for every planar graph $H$, $$c(H) = (1+o(1))\cdot\max\left\{\frac{V(H)}{2},V(H)  \alpha (H)\right\},$$ extending recent results of Haslegrave, Kim and Liu (2020). We also show that an asymptotically tight bound on the extremal function of graphs in minorclosed families proposed by Haslegrave, Kim and Liu (2020) is equivalent to a well studied open weakening of Hadwiger's conjecture.
 Publication:

arXiv eprints
 Pub Date:
 July 2021
 DOI:
 10.48550/arXiv.2107.08658
 arXiv:
 arXiv:2107.08658
 Bibcode:
 2021arXiv210708658H
 Keywords:

 Mathematics  Combinatorics
 EPrint:
 Advances in Combinatorics, 2022:5, 43pp