English

On Ramsey goodness of $K_{2,n}$ versus cycles

Combinatorics 2026-05-11 v2

Abstract

A graph GG is called HH-good if R(G,H)=(G1)(χ(H)1)+σ(H)R(G,H)=(|G|-1)(\chi(H)-1)+\sigma(H), where σ(H)\sigma(H) denotes the size of the smallest color class in a χ(H)\chi(H)-coloring of HH. In Ramsey theory, it is an interesting problem to study whether a graph GG is HH-good or not. In this article, we study the Ramsey goodness of the pair (K2,n,Cm)(K_{2,n},C_m), which naturally lies between the classical star-cycle and book-cycle problems. We prove that \begin{equation*} R(K_{2,n},C_{\{m,m+1\}})=m+1. \end{equation*} for all m2n+1m\ge 2n+1, and consequently establish that \begin{equation*} R(K_{2,n},C_{m})=m+1. \end{equation*} for all m3n+4m\ge 3n+4. This proves that CmC_m is K2,nK_{2,n}-good in this range and improves a particular case of a result on the Ramsey goodness by Pokrovskiy and Sudakov. Further, we provide a construction of a graph that disproves the CmC_{m}-goodness of K2,nK_{2,n} for all even mm satisfying nm+2n\geq m+2.

Keywords

Cite

@article{arxiv.2605.06253,
  title  = {On Ramsey goodness of $K_{2,n}$ versus cycles},
  author = {Abisek Dewan and Sayan Gupta and Rajiv Mishra},
  journal= {arXiv preprint arXiv:2605.06253},
  year   = {2026}
}

Comments

16 pages, 3 figures