English

Directed graphs with lower orientation Ramsey thresholds

Combinatorics 2024-06-18 v2

Abstract

We investigate the threshold pH=pH(n)p_{\vec H}=p_{\vec H}(n) for the Ramsey-type property G(n,p)HG(n,p)\to \vec H, where G(n,p)G(n,p) is the binomial random graph and GHG\to\vec H indicates that every orientation of the graph GG contains the oriented graph H\vec H as a subdigraph. Similarly to the classical Ramsey setting, the upper bound pHCn1/m2(H)p_{\vec H}\leq Cn^{-1/m_2(\vec H)} is known to hold for some constant C=C(H)C=C(\vec H), where m2(H)m_2(\vec H) denotes the maximum 22-density of the underlying graph HH of H\vec H. While this upper bound is indeed the threshold for some H\vec H, this is not always the case. We obtain examples arising from rooted products of orientations of sparse graphs (such as forests, cycles and, more generally, subcubic {K3,K3,3}\{K_3,K_{3,3}\}-free graphs) and arbitrarily rooted transitive triangles.

Keywords

Cite

@article{arxiv.2211.07033,
  title  = {Directed graphs with lower orientation Ramsey thresholds},
  author = {Gabriel Ferreira Barros and Bruno Pasqualotto Cavalar and Yoshiharu Kohayakawa and Guilherme Oliveira Mota and Tássio Naia},
  journal= {arXiv preprint arXiv:2211.07033},
  year   = {2024}
}

Comments

12 pages, 1 figure. To appear in RAIRO-Operations Research

R2 v1 2026-06-28T05:45:56.945Z