Lévy hierarchy

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English

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Etymology

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Introduced by Azriel Lévy in 1965.

Proper noun

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the Lévy hierarchy

  1. (set theory, logic) A hierarchy of formulas in the formal language of the Zermelo-Fraenkel set theory. Its first level contains only formulas with no unbounded quantifiers and is denoted by . Subsequent levels are given by finding a formula in prenex normal form which is provably equivalent over ZFC, and counting the number of changes of quantifiers.