Ramsey number

In combinatorial mathematics, Ramsey's theorem states that one will find monochromatic cliques in any edge labelling (with colours) of a sufficiently large complete graph. To demonstrate the theorem for two colours (say, blue and red), let r and s be any two positive integers. Ramsey's theorem states that there exists a least positive integer R(r, s) for which every blue-red edge colouring of the complete graph on R(r, s) vertices contains a blue clique on r vertices or a red clique on s vertices. (Here R(r, s) signifies an integer that depends on both r and s.)

Ramsey's theorem is a foundational result in combinatorics. The first version of this result was proved by F. P. Ramsey. This initiated the combinatorial theory now called Ramsey theory, that seeks regularity amid disorder: general conditions for the existence of substructures with regular properties. In this application it is a question of the existence of monochromatic subsets, that is, subsets of connected edges of just one colour.

An extension of this theorem applies to any finite number of colours, rather than just two. More precisely, the theorem states that for any given number of colours, c, and any given integers n₁, …, n_c, there is a number, R(n₁, …, n_c), such that if the edges of a complete graph of order R(n₁, ..., n_c) are coloured with c different colours, then for some i between 1 and c, it must contain a complete subgraph of order n_i whose edges are all colour i. The special case above has c = 2 (and n₁ = r and n₂ = s).

In the 2-colour case, an arbitrary simple graph G = (V, E) can be identified with the complete graph on the vertex set V whose edges are coloured with two colours (all the edges corresponding to those in E receive one colour and all the other edges receive the other colour.) This permits talking about Ramsey's theorem using "connected" and "non-connected" terminology instead of colours, but this language does not generalize to a greater number of colours. In the following example, the formula R(3, 3) provides a solution to the question which asks for the minimum number of vertices a graph must contain in order to ensure that either:

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