English

On some three color Ramsey numbers for paths, cycles, stripes and stars

Combinatorics 2017-07-24 v1

Abstract

For given graphs G1,G2,...,Gk,k2G_{1}, G_{2}, ... , G_{k}, k \geq 2, the multicolor Ramsey number R(G1,G2,...,Gk)R(G_{1}, G_{2}, ... , G_{k}) is the smallest integer nn such that if we arbitrarily color the edges of the complete graph of order nn with kk colors, then it always contains a monochromatic copy of GiG_{i} colored with ii, for some 1ik1 \leq i \leq k. The bipartite Ramsey number b(G1,,Gk)b(G_1, \cdots, G_k) is the least positive integer bb such that any coloring of the edges of Kb,bK_{b,b} with kk colors will result in a monochromatic copy of bipartite GiG_i in the ii-th color, for some ii, 1ik1 \le i \le k. There is very little known about R(G1,,Gk)R(G_{1},\ldots, G_{k}) even for very special graphs, there are a lot of open cases. In this paper, by using bipartite Ramsey numbers we obtain the exact values of some multicolor Ramsey numbers. We show that for sufficiently large n0n_{0} and three following cases: 1. n1=2sn_{1}=2s, n2=2mn_{2}=2m and m1<2sm-1<2s, 2. n1=n2=2sn_{1}=n_{2}=2s, 3. n1=2s+1n_{1}=2s+1, n2=2mn_{2}=2m and s<m1<2s+1s<m-1<2s+1, we have R(Cn0,Pn1,Pn2)=n0+n12+n222.R(C_{n_0}, P_{n_{1}},P_{n_{2}}) = n_0 + \Big \lfloor \frac{n_1}{2} \Big \rfloor + \Big \lfloor \frac{n_2}{2} \Big \rfloor -2. We prove that R(Pn,kK2,kK2)=n+2k2R(P_n,kK_{2},kK_{2})=n+2k-2 for large nn. In addition, we prove that for even kk, R((k1)K2,Pk,Pk)=3k4R((k-1)K_{2},P_{k},P_{k})=3k-4. For s<m1<2s+1s < m-1<2s+1 and tm+s1t\geq m+s-1, we obtain that R(tK2,P2s+1,P2m)=s+m+2t2R(tK_{2},P_{2s+1},P_{2m})=s+m+2t-2 where PkP_{k} is a path on kk vertices and tK2tK_{2} is a matching of size tt. We also provide some new exact values or generalize known results for other multicolor Ramsey numbers of paths, cycles, stripes and stars versus other graphs.

Keywords

Cite

@article{arxiv.1707.06955,
  title  = {On some three color Ramsey numbers for paths, cycles, stripes and stars},
  author = {Farideh Khoeini and Tomasz Dzido},
  journal= {arXiv preprint arXiv:1707.06955},
  year   = {2017}
}