English

A counterexample to Hickingbotham's conjecture about $k$-ghost-edges

Combinatorics 2026-02-04 v1

Abstract

Fix kNk\in \mathbb{N} and let GG be a connected graph with tw(G)ktw(G)\leq k. We say that xyE(Gc)xy\in E(G^c) is a {\em kk-ghost-edge} of GG if for every tree decomposition (T,\cB)(T,\cB) of GG with width at most kk, the set {x,y}\{x,y\} is contained in a bag of (T,\cB)(T,\cB). Although a kk-ghost-edge of GG is not an edge of GG, but it behaves like real edges with respect to tree decomposition of GG with width at most kk. For any graph GG with treewidth kk and xyE(Gc)xy\in E(G^c), when there are at least k+1k+1 internally vertex disjoint (x,y)(x,y)-paths, Hickingbotham proved that xyxy is a kk-ghost-edge of GG; while when there are at most kk internally vertex disjoint (x,y)(x,y)-paths, he conjectured that it is not a kk-ghost-edge of GG. In this paper, we prove that this conjecture is wrong.

Cite

@article{arxiv.2602.03016,
  title  = {A counterexample to Hickingbotham's conjecture about $k$-ghost-edges},
  author = {Rong Chen},
  journal= {arXiv preprint arXiv:2602.03016},
  year   = {2026}
}
R2 v1 2026-07-01T09:33:21.182Z