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In set theory, a branch of mathematics, the condensation lemma is a result about sets in the constructible universe. It states that if ''X'' is a transitive set and is an elementary submodel of some level of the constructible hierarchy Lα, that is, , then in fact there is some ordinal such that . More can be said: If ''X'' is not transitive, then its transitive collapse is equal to some , and the hypothesis of elementarity can be weakened to elementarity only for formulas which are in the Lévy hierarchy. Also, the assumption that ''X'' be transitive automatically holds when . The lemma was formulated and proved by Kurt Gödel in his proof that the axiom of constructibility implies GCH. == References == * (theorem II.5.2 and lemma II.5.10) 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Condensation lemma」の詳細全文を読む スポンサード リンク
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