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Projective hierarchy

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We may define the set of "first projective" subsets of R^n to be the set of all subsets which are projections of Borel subsets of R^n+1; then we may define "second projective" sets as projections of first projective sets or complements thereof, and so on. A set is said to be projective if it belongs to some level of this hierarchy. Template:Substub

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