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It follows from Ramsey 's theorem that for any graph G there exists a least integer r ( G ) , the Ramsey number of G , such that any complete graph on at least r ( G ) vertices whose edges are coloured red or blue contains a monochromatic copy of G. For instance , the Ramsey number of a triangle is 6 : no matter how the edges of a complete graph on six vertices are colored red or blue , there is always either a red triangle or a blue triangle .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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