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Lévy hierarchy. In DICTIOUS you will not only get to know all the dictionary meanings for the word
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English
Etymology
Introduced by Azriel Lévy in 1965.
Proper noun
the Lévy hierarchy
- (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.