English

Ramsey numbers for multiple copies of sparse graphs

Combinatorics 2022-12-06 v1

Abstract

For a graph HH and an integer nn, we let nHnH denote the disjoint union of nn copies of HH. In 1975, Burr, Erd\H{o}s, and Spencer initiated the study of Ramsey numbers for nHnH, one of few instances for which Ramsey numbers are now known precisely. They showed that there is a constant c=c(H)c = c(H) such that r(nH)=(2Hα(H))n+cr(nH) = (2|H| - \alpha(H))n + c, provided nn is sufficiently large. Subsequently, Burr gave an implicit way of computing cc and noted that this long term behaviour occurs when nn is triply exponential in H|H|. Very recently, Buci\'{c} and Sudakov revived the problem and established an essentially tight bound on nn by showing r(nH)r(nH) follows this behaviour already when the number of copies is just a single exponential. We provide significantly stronger bounds on nn in case HH is a sparse graph, most notably of bounded maximum degree. These are relatable to the current state of the art bounds on r(H)r(H) and (in a way) tight. Our methods rely on a beautiful classic proof of Graham, R\"{o}dl, and Ruci\'{n}ski, with the emphasis on developing an efficient absorbing method for bounded degree graphs.

Keywords

Cite

@article{arxiv.2212.02455,
  title  = {Ramsey numbers for multiple copies of sparse graphs},
  author = {Aurelio Sulser and Miloš Trujić},
  journal= {arXiv preprint arXiv:2212.02455},
  year   = {2022}
}
R2 v1 2026-06-28T07:22:43.444Z