Ramsey’s theorem
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Ramsey’s theorem
Ramsey theorem在<<Introduction to the Theory of Computation>>的习题0.14。
英文版本为，
Let G be a graph. A clique in G is a subgraph in which every two nodes are connected by an edge. An anti-clique, also called an independent set, is a subgraph in which every two nodes are not connected by an edge. Show that every graph with n nodes contains either a clique or an anti-clique with at least nodes
翻译如下，
表示一个图。一个小集团(clique)是图G的一个子图，这个子图任意的两个节点都有一条边关联。一个反集团(anti-clique)，也叫独立集，它里面没有一对节点是相连的。证明每一个有n个节点的图，总会包含至少有
大小的小集团或者反集团。
小集团，反集团听起来很难理解，其实Ramsey theorem有一个更形象的名字，叫友谊定理。
我们可以认为相连的两个节点，相当于两个互相认识的人。不相连的两个节点，相当于两个人不认识。所以上面的定理，可以表述为任意一个有n个人的房间里面，总会有至少 个人相互认识，或者相互不认识。
比如房间里面有16个人，以下情况总会有一个成立，
- 至少有2个人互相认识
- 至少有2个人互相不认识
额，这不是很明显吗？所以应该关注的问题是，如何证明这个定理。
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