Extremal density for sparse minors and subdivisions

Abstract

Abstract We prove an asymptotically tight bound on the extremal density guaranteeing subdivisions of bounded-degree bipartite graphs with a mild separability condition. As corollaries, we answer several questions of Reed and Wood on embedding sparse minors. Among others, $(1+o(1))t^2$ average degree is sufficient to force the $t\times t$ grid as a topological minor;$(3/2+o(1))t$ average degree forces every  $t$-vertex planar graph as a minor, and the constant $3/2$ is optimal, furthermore, surprisingly, the value is the same for $t$-vertex graphs embeddable on any fixed surface;a universal bound of $(2+o(1))t$ on average degree forcing every  $t$-vertex graph in any nontrivial minor-closed family as a minor, and the constant 2 is best possible by considering graphs with given treewidth.