English

On off-diagonal ordered Ramsey numbers of nested matchings

Combinatorics 2022-10-12 v2 Discrete Mathematics

Abstract

For two graphs G<G^< and H<H^< with linearly ordered vertex sets, the ordered Ramsey number r<(G<,H<)r_<(G^<,H^<) is the minimum NN such that every red-blue coloring of the edges of the ordered complete graph on NN vertices contains a red copy of G<G^< or a blue copy of H<H^<. For a positive integer nn, a nested matching NMn<NM^<_n is the ordered graph on 2n2n vertices with edges {i,2ni+1}\{i,2n-i+1\} for every i=1,,ni=1,\dots,n. We improve bounds on the ordered Ramsey numbers r<(NMn<,K3<)r_<(NM^<_n,K^<_3) obtained by Rohatgi, we disprove his conjecture by showing 4n+1r<(NMn<,K3<)(3+5)n4n+1 \leq r_<(NM^<_n,K^<_3) \leq (3+\sqrt{5})n for every n6n \geq 6, and we determine the numbers r<(NMn<,K3<)r_<(NM^<_n,K^<_3) exactly for n=4,5n=4,5. As a corollary, this gives stronger lower bounds on the maximum chromatic number of kk-queue graphs for every k3k \geq 3. We also prove r<(NMm<,Kn<)=Θ(mn)r_<(NM^<_m,K^<_n)=\Theta(mn) for arbitrary mm and nn. We expand the classical notion of Ramsey goodness to the ordered case and we attempt to characterize all connected ordered graphs that are nn-good for every nNn\in\mathbb{N}. In particular, we discover a new class of ordered trees that are nn-good for every nNn \in \mathbb{N}, extending all the previously known examples.

Keywords

Cite

@article{arxiv.2201.07637,
  title  = {On off-diagonal ordered Ramsey numbers of nested matchings},
  author = {Martin Balko and Marian Poljak},
  journal= {arXiv preprint arXiv:2201.07637},
  year   = {2022}
}

Comments

17 pages, 7 figures, minor revisions

R2 v1 2026-06-24T08:55:17.689Z