$K_h$-Minor-Free Graphs Have Balanced Separators of Size $O(h\sqrt{n})$

Abstract

We show that every $K_h$-minor-free graph with $n$ vertices has a balanced separator of size $O(h\sqrt{n})$ for any parameter $h \in \mathbb{N}$, resolving the conjecture posed by Alon, Seymour, and Thomas [AST90] in 1990. The proof is elementary, building on the combinatorial framework by Le [Le26]. A key new idea of the proof is the existence of a short hitting walk on vertex expanders: given $k$ large (not necessarily disjoint) subsets of vertices, there is a walk of length $O(k/ψ(G))$ hitting every subset, where $ψ(G)$ is the vertex expansion. This technical result substantially improves on an analogous lemma by Alon, Seymour, and Thomas [AST90] in an important parameter setting, where the previous lower bound [Kot00] does not apply.