English

New bounds for Ramsey numbers $R(K_k-e,K_l-e)$

Combinatorics 2021-07-12 v1 Discrete Mathematics

Abstract

Let R(H1,H2)R(H_1,H_2) denote the Ramsey number for the graphs H1,H2H_1, H_2, and let JkJ_k be KkeK_k{-}e. We present algorithms which enumerate all circulant and block-circulant Ramsey graphs for different types of graphs, thereby obtaining several new lower bounds on Ramsey numbers including: 49R(K3,J12)49 \leq R(K_3,J_{12}), 36R(J4,K8)36 \leq R(J_4,K_8), 43R(J4,J10)43 \leq R(J_4,J_{10}), 52R(K4,J8)52 \leq R(K_4,J_8), 37R(J5,J6)37 \leq R(J_5,J_6), 43R(J5,K6)43 \leq R(J_5,K_6), 65R(J5,J7)65\leq R(J_5,J_7). We also use a gluing strategy to derive a new upper bound on R(J5,J6)R(J_5,J_6). With both strategies combined, we prove the value of two Ramsey numbers: R(J5,J6)=37R(J_5,J_6)=37 and R(J5,J7)=65R(J_5,J_7)=65. We also show that the 64-vertex extremal Ramsey graph for R(J5,J7)R(J_5,J_7) is unique. Furthermore, our algorithms also allow to establish new lower bounds and exact values on Ramsey numbers involving wheel graphs and complete bipartite graphs, including: R(W7,W4)=21R(W_7,W_4) = 21, R(W7,W7)=19R(W_7,W_7) = 19, R(K3,4,K3,4)=25R(K_{3,4},K_{3,4}) = 25, and R(K3,5,K3,5)=33R(K_{3,5}, K_{3,5})=33.

Keywords

Cite

@article{arxiv.2107.04460,
  title  = {New bounds for Ramsey numbers $R(K_k-e,K_l-e)$},
  author = {Jan Goedgebeur and Steven Van Overberghe},
  journal= {arXiv preprint arXiv:2107.04460},
  year   = {2021}
}

Comments

17 pages

R2 v1 2026-06-24T04:02:37.923Z