
 Union (set Theory) Encyclopedia Article, Definition, History, Biography (Site not responding. Last check: 20071106) 
  In set theory and other branches of mathematics, the union of a collection of sets is the set that contains everything that belongs to any of the sets, but nothing else. 
  For example, the union of the sets {1, 2, 3} and {2, 3, 4} is {1, 2, 3, 4}. 
  That this union of M is a set no matter how large a set M itself might be, is the content of the axiom of union in axiomatic set theory. 
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