Ramsey (1903~ 1930) is a British mathematical logician. He generalized the pigeon hole principle and got the generalized pigeon hole principle, also known as Ramsey theorem. The content of Ramsey's theorem (in a narrow sense): at least three of any six people know or at least three people don't know. The proof is as follows: Let these six people be six points A, B, C, D, E and F, and five line segments AB, AC, AD, AE and AF can be drawn from point A.. Suppose: if two people know each other, let the line segment formed by these two people be red; If two people don't know each other, let the line formed by these two people be blue. According to pigeonhole principle, at least three of these five line segments are of the same color. Let AB, AC and AD be red. If BC or CD is red, the conclusion is obviously valid. If BC and CD are blue, if BD is red, there must be three people who know each other; If BD is blue, there must be three people who don't know each other.

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