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Set-theoretic definition of natural numbers : ウィキペディア英語版 | Set-theoretic definition of natural numbers
Several ways have been proposed to define the natural numbers using set theory. == Contemporary standard == In standard, Zermelo–Fraenkel (ZF) set theory the natural numbers are defined recursively by 0 = (the empty set) and ''n'' + 1 = ''n'' ∪ . Then ''n'' = for each natural number ''n''. The first few numbers defined this way are 0 = , 1 = = , 2 = = }}, 3 = = ,}}}}. The set ''N'' of natural numbers is defined as the smallest set containing 0 and closed under the successor function ''S'' defined by ''S(n)'' = ''n'' ∪ . (For the existence of such a set we need an axiom of infinity.) The structure ⟨''N'',0,''S''⟩ is a model of Peano arithmetic. The set ''N'' and its elements, when constructed this way, are examples of von Neumann ordinals.
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