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'''Set theory''' is a branch of [[mathematics]] dealing with collections of objects.  It revolutionized mathematics and made possible enormous new insights.

'''Set theory''' is a branch of [[mathematics]] dealing with collections of objects, called [[set]]s.  It revolutionized mathematics and made possible enormous new insights.

*The language of set theory is based on a single fundamental relation, called membership. We say that A is a member of B (in symbols A ∈ B), or that the set B contains A as its element.<ref>The mathematician Giuseppe Peano created the symbol ∈ in 1889 to mean "is an element of," from the Greek epsilon that is the first letter of εἰμί, which means "I am."</ref> The understanding is that a set is determined by its elements; in other words, two sets are deemed equal if they have exactly the same elements. <ref>http://plato.stanford.edu/entries/set-theory/</ref>

*The language of set theory is based on a single fundamental relation, called membership. We say that A is a member of B (in symbols A ∈ B), or that the set B contains A as its element.<ref>The mathematician Giuseppe Peano created the symbol ∈ in 1889 to mean "is an element of," from the Greek epsilon that is the first letter of εἰμί, which means "I am."</ref> The understanding is that a set is determined by its elements; in other words, two sets are deemed equal if they have exactly the same elements. <ref>http://plato.stanford.edu/entries/set-theory/</ref>