English

Off-diagonal ordered Ramsey numbers of matchings

Combinatorics 2018-08-14 v1

Abstract

For ordered graphs GG and HH, the ordered Ramsey number r<(G,H)r_<(G,H) is the smallest nn such that every red/blue edge coloring of the complete graph on vertices {1,,n}\{1,\dots,n\} contains either a blue copy of GG or a red copy of HH, where the embedding must preserve the relative order of vertices. One number of interest, first studied by Conlon, Fox, Lee, and Sudakov, is the "off-diagonal" ordered Ramsey number r<(M,K3)r_<(M, K_3), where MM is an ordered matching on nn vertices. In particular, Conlon et al. asked what asymptotic bounds (in nn) can be obtained for maxr<(M,K3)\max r_<(M, K_3), where the maximum is over all ordered matchings MM on nn vertices. The best-known upper bound is O(n2/logn)O(n^2/\log n), whereas the best-known lower bound is Ω((n/logn)4/3)\Omega((n/\log n)^{4/3}), and Conlon et al. hypothesize that r<(M,K3)=O(n2ϵ)r_<(M, K_3) = O(n^{2-\epsilon}) for every ordered matching MM. We resolve two special cases of this conjecture. We show that the off-diagonal ordered Ramsey numbers for matchings in which edges do not cross are nearly linear. We also prove a truly sub-quadratic upper bound for random matchings with interval chromatic number 22.

Keywords

Cite

@article{arxiv.1808.04025,
  title  = {Off-diagonal ordered Ramsey numbers of matchings},
  author = {Dhruv Rohatgi},
  journal= {arXiv preprint arXiv:1808.04025},
  year   = {2018}
}

Comments

15 pages, 3 figures

R2 v1 2026-06-23T03:31:31.760Z