On cycle—Complete graph ramsey numbers

A new upper bound is given for the cycle-complete graph Ramsey number r(C_m, K_n), the smallest order for a graph which forces it to contain either a cycle of order m or a set of n independent vertices. Then, another cycle-complete graph Ramsey number is studied, namely r(?C_m, K_n) the smallest order for a graph which forces it to contain either a cycle of order / for some / satisfying 3?/?m or a set of n independent vertices. We obtain the exact value of r(?C_m K_n) for all m > n and an upper bound which applies when m is large in comparison with log n.     

Ramsey functions involving with large

For fixed integers m,k2, it is shown that the k-color Ramsey number r_k(K_m,n) and the bipartite Ramsey number b_k(m,n) are both asymptotically equal to k^mn as n→∞, and that for any graph H on m vertices, the two-color Ramsey number is at most (1+o(1))n^m+1/(logn)^m-1. Moreover, the order of magnitude of is proved to be n^m+1/(logn)^m if H≠K_m as n→∞.     

Off‐diagonal and asymptotic results on the Ramsey number r(K2,m,K2,n)

An upper bound on the Ramsey number r(K_2,n‐s,K_2,n) where s ≥ 2 is presented. Considering certain r(K_2,n‐s,K_2,n)‐colorings obtained from strongly regular graphs, we additionally prove that this bound matches the exact value of r(K_2,n‐s,K_2,n) in infinitely many cases if holds. Moreover, the asymptotic behavior of r(K_2,m,K_2,n) is studied for n being sufficiently large depending on m. We conclude with a table of all known Ramsey numbers r(K_2,m,K_2,n) where m,n ≤ 10. © 2003 Wiley Periodicals, Inc. J Graph Theory 43: 252–268, 2003     

Ramsey numbers for multiple copies of complete graphs

The Ramsey numbers r(m₁K_p, …, m_cK) are calculated to within bounds which are independent of m₁, …, m_c when c > 2 and p₁, …, p_c > 2.     

The Ramsey numbers r(Pm, Kn)

This note evaluates the Ramsey numbers r(P_m,K_n), and discusses developments in 0 generalized Ramsey theory for graphs.     

On irredundant Ramsey numbers for graphs

The irredundant Ramsey number s(m, n) is the smallest p such that in every two-coloring of the edges of K_p using colors red (R) and blue (B), either the blue graph contains an m-element irredundant set or the red graph contains an n-element irredundant set. We develop techniques to obtain upper bounds for irredundant Ramsey numbers of the form s(3, n) and prove that 18 ≤ s(3,7) ≤ 19.     

All cycle‐complete graph Ramsey numbers r(Cm,K6)

The cycle‐complete graph Ramsey number r(C_m, K_n) is the smallest integer N such that every graph G of order N contains a cycle C_m on m vertices or has independence number α(G) ≥ n. It has been conjectured by Erd?s, Faudree, Rousseau and Schelp that r(C_m, K_n) = (m ? 1) (n ? 1) + 1 for all m ≥ n ≥ 3 (except r(C₃, K₃) = 6). This conjecture holds for 3 ≤ n ≤ 5. In this paper we will present a proof for n = 6 and for all n ≥ 7 with m ≥ n² ? 2n. © 2003 Wiley Periodicals, Inc. J Graph Theory 44: 251–260, 2003     

ASYMPTOTIC BOUNDS FOR IRREDUNDANT RAMSEY NUMBERS

Abstract

The irredundant Ramsey number s(m,n) is the smallest N such that in every red-blue colouring of the edges of K_N , either the blue graph contains an m-element irredundant set or the red graph contains an n-element irredundant set. We prove an asymptotic lower bound for s(m, n).     

Gallai–Ramsey numbers for cycles

For given graphs G and H and an integer k, the Gallai–Ramsey number is defined to be the minimum integer n such that, in any k coloring of the edges of K_n, there exists a subgraph isomorphic to either a rainbow coloring of G or a monochromatic coloring of H. In this work, we consider Gallai–Ramsey numbers for the case when G=K₃ and H is a cycle of a fixed length.     

On the Ramsey number of trees versus graphs with large clique number

Chvátal established that r(T_m, K_n) = (m – 1)(n – 1) + 1, where T_m is an arbitrary tree of order m and K_n is the complete graph of order n. This result was extended by Chartrand, Gould, and Polimeni who showed K_n could be replaced by a graph with clique number n and order n + 1 provided n ≧ 3 and m ≧ 3. We further extend these results to show that K_n can be replaced by any graph on n + 2 vertices with clique number n, provided n ≧ 5 and m ≧ 4. We then show that further extensions, in particular to graphs on n + 3 vertices with clique number n are impossible. We also investigate the Ramsey number of trees versus complete graphs minus sets of independent edges. We show that r(T_m, K_n –tK₂) = (m – 1)(n – t – 1) + 1 for m ≧ 3, n ≧ 6, where T_m is any tree of order m except the star, and for each t, O ≦ t ≦ [(n – 2)/2].     

Bounds on Ramsey Numbers of Certain Complete Bipartite Graphs

Bounds on the Ramsey number r(K_l,m,K_l,n), where we may assume l ≤ m ≤ n, are determined for 3 ≤ l ≤ 5 and m ≈ n. Particularly, for m = n the general upper bound on r(K_l,n, K_l,n) due to Chung and Graham is improved for those l. Moreover, the behavior of r(K_3,m, K_3,n) is studied for m fixed and n sufficiently large.     

Star-critical Ramsey numbers

The graph Ramsey numberR(G,H) is the smallest integer r such that every 2-coloring of the edges of K_r contains either a red copy of G or a blue copy of H. We find the largest star that can be removed from K_r such that the underlying graph is still forced to have a red G or a blue H. Thus, we introduce the star-critical Ramsey numberr_∗(G,H) as the smallest integer k such that every 2-coloring of the edges of K_r−K_1,r−1−k contains either a red copy of G or a blue copy of H. We find the star-critical Ramsey number for trees versus complete graphs, multiple copies of K₂ and K₃, and paths versus a 4-cycle. In addition to finding the star-critical Ramsey numbers, the critical graphs are classified for R(T_n,K_m), R(nK₂,mK₂) and R(P_n,C₄).     

On ramsey numbers for books

For n = 1, 2, …, let B_n = K₂ + K?_n. We pose the problem of determining the Ramsey numbers r(B_m, B_n) and demonstrate that in many cases critical colorings are avialable from known examples of strongly regular graphs.     

Multipartite Ramsey numbers for odd cycles

In this paper we study multipartite Ramsey numbers for odd cycles. We formulate the following conjecture: Let n≥5 be an arbitrary positive odd integer; then, in any two‐coloring of the edges of the complete 5‐partite graph K((n?1)/2, (n?1)/2, (n?1)/2, (n?1)/2, 1) there is a monochromatic C_n, a cycle of length n. This roughly says that the Ramsey number for C_n (i.e. 2n?1 ) will not change (somewhat surprisingly) if four large “holes” are allowed. Note that this would be best possible as the statement is not true if we delete from K_2n?1 the edges within a set of size (n+ 1)/2. We prove an approximate version of the above conjecture. © 2009 Wiley Periodicals, Inc. J Graph Theory 61:12‐21, 2009     

Some properties of Ramsey numbers

In this paper, some properties of Ramsey numbers are studied, and the following results are presented.

1. (1) For any positive integers k₁, k₂, …, k_m l₁, l₂, …, l_m (m> 1), we have

.

2. (2) For any positive integers k₁, k₂, …, k_m, l₁, l₂, …, l_n , we have

. Based on the known results of Ramsey numbers, some results of upper bounds and lower bounds of Ramsey numbers can be directly derived by those properties.

     

Asymptotic bounds for irredundant and mixed Ramsey numbers

The irredundant Ramsey number s(m, n) is the smallest N such that in every red-blue coloring of the edges of K_N, either the blue graph contains an m-element irredundant set or the red graph contains an n-element irredundant set. The definition of the mixed Ramsey number t(m, n) differs from s(m, n) in that the n-element irredundant set is replaced by an n-element independent set. We prove asymptotic lower bounds for s(n, n) and t(m, n) (with m fixed and n large) and a general upper bound for t(3, n). © 1993 John Wiley & Sons, Inc.     

On the Erd s-Ginzburg-Ziv theorem and the Ramsey numbers for stars and matchings

A link between Ramsey numbers for stars and matchings and the Erd s-Ginzburg-Ziv theorem is established. Known results are generalized. Among other results we prove the following two theorems. Theorem 5. Let m be an even integer. If c : e (K_2m−1)→{0, 1,…, m−1} is a mapping of the edges of the complete graph on 2m−1 vertices into {0, 1,…, m−1}, then there exists a star K_1,m in K_2m−1 with edges e₁, e₂,…, e_m such that c(e₁)+c(e₂)++c(e_m)≡0 (mod m). Theorem 8. Let m be an integer. If c : e(K^r_(r+1)m−1)→{0, 1,…, m−1} is a mapping of all the r-subsets of an (r+1)m−1 element set S into {0, 1,…, m−1}, then there are m pairwise disjoint r-subsets Z₁, Z₂,…, Z_m of S such that c(Z₁)+c(Z₂)++c(Z_m)≡0 (mod m).     

Canonical Pattern Ramsey Numbers

A color pattern is a graph whose edges have been partitioned into color classes. A family of color patterns is a Ramsey family provided there is some sufficiently large integer N such that in any edge coloring of the complete graph K_N there is an (isomorphic) copy of at least one of the patterns from . The smallest such N is the Ramsey number of the family . The classical Canonical Ramsey theorem of Erds and Rado asserts that the family of color patterns is a Ramsey family if it consists of monochromatic, rainbow (totally multicolored) and lexically colored complete graphs. In this paper we treat the asymmetric case by studying the Ramsey number of families containing a rainbow triangle, a lexically colored complete graph and a fixed arbitrary monochromatic graph. In particular we give asymptotically tight bounds for the Ramsey number of a family consisting of rainbow and monochromatic triangle and a lexically colored K_N. Among others, we prove some canonical Ramsey results for cycles.     

Large Kr‐free subgraphs in Ks‐free graphs and some other Ramsey‐type problems

In this paper we present three Ramsey‐type results, which we derive from a simple and yet powerful lemma, proved using probabilistic arguments. Let 3 ≤ r < s be fixed integers and let G be a graph on n vertices not containing a complete graph K_s on s vertices. More than 40 years ago Erd?s and Rogers posed the problem of estimating the maximum size of a subset of G without a copy of the complete graph K_r. Our first result provides a new lower bound for this problem, which improves previous results of various researchers. It also allows us to solve some special cases of a closely related question posed by Erd?s. For two graphs G and H, the Ramsey number R(G, H) is the minimum integer N such that any red‐blue coloring of the edges of the complete graph K_N, contains either a red copy of G or a blue copy of H. The book with n pages is the graph B_n consisting of n triangles sharing one edge. Here we study the book‐complete graph Ramsey numbers and show that R(B_n, K_n) ≤ O(n³/log^3/2n), improving the bound of Li and Rousseau. Finally, motivated by a question of Erd?s, Hajnal, Simonovits, Sós, and Szemerédi, we obtain for all 0 < δ < 2/3 an estimate on the number of edges in a K₄‐free graph of order n which has no independent set of size n^1‐δ. © 2004 Wiley Periodicals, Inc. Random Struct. Alg., 2005     

A class of self-complementary graphs and lower bounds of some ramsey numbers

A method is described of constructing a class of self-complementary graphs, that includes a self-complementary graph, containing no K₅, with 41 vertices and a self-complementary graph, containing no K₇, with 113 vertices. The latter construction gives the improved Ramsey number lower bound r(7, 7) ≥ 114.