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In mathematical logic, the quantifier rank of a formula is the depth of nesting of its quantifiers. It plays an essential role in model theory. Notice that the quantifier rank is a property of the formula itself (i.e. the expression in a language). Thus two logically equivalent formulae can have different quantifier ranks, when they express the same thing in different ways. ==Definition== Quantifier Rank of a Formula in First-order language (FO) Let φ be a FO formula. The quantifier rank of φ, written qr(φ), is defined as * , if φ is atomic. * . * . * . Remarks * We write FO() for the set of all first-order formulas φ with . * Relational FO() (without function symbols) is always of finite size, i.e. contains a finite number of formulas * Notice that in Prenex normal form the Quantifier Rank of φ is exactly the number of quantifiers appearing in φ. Quantifier Rank of a higher order Formula * For Fixpoint logic, with a least fix point operator LFP: : qr(()y) = 1 + qr( φ) ... 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「Quantifier rank」の詳細全文を読む スポンサード リンク
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