English

Compatible Hamilton cycles in graphs with large minimum degree

Combinatorics 2026-03-24 v1

Abstract

The renowned theorem of Dirac states that if GG is a graph with minimum degree at least n/2n/2 then GG has a Hamilton cycle. A natural generalisation asks what properties of an edge-colouring of GG guarantee the existence of a properly edge-coloured Hamilton cycle in GG. This concept can be further generalised as follows: an \emph{incompatibility system} for GG is a set~F\mathcal{F} of `forbidden' pairs of adjacent edges, that is, F{{uv,vw}(E(G)2)}\mathcal{F}\subseteq \{\{uv,vw\}\in \binom{E(G)}2\}. A cycle in GG is then \emph{compatible} if no two of its edges form a pair in F\mathcal{F}. The system F\mathcal{F} is called \emph{μn\mu n-bounded} if for all vV(G)v\in V(G) and uvE(G)uv\in E(G), there are at most μn\mu n pairs {uv,vw}F\{uv,vw\}\in \mathcal{F}. How small must μ\mu be to guarantee the existence of a compatible Hamilton cycle in GG? Krivelevich, Lee and Sudakov showed that μ=1016\mu=10^{-16} suffices (for nn large), while an example of Bollob\'as and Erd\H{o}s shows that μ1/4\mu\leq 1/4 is necessary. We significantly reduce this gap for large graphs of minimum degree at least (1/2+ε)n(1/2+\varepsilon)n, by showing that μ=1/8\mu=1/8 suffices but μ1/6\mu\leq 1/6 is necessary for such graphs. In fact, we give more precise bounds which are functions of δ(G)/n\delta(G)/n.

Keywords

Cite

@article{arxiv.2603.21984,
  title  = {Compatible Hamilton cycles in graphs with large minimum degree},
  author = {Natalie Behague and Francesco Di Braccio and Bertille Granet and Allan Lo},
  journal= {arXiv preprint arXiv:2603.21984},
  year   = {2026}
}

Comments

23 pages

R2 v1 2026-07-01T11:33:20.663Z