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Descriptive set theory
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== Analytic and coanalytic sets == Just beyond the Borel sets in complexity are the [[analytic set]]s and [[coanalytic set]]s. A subset of a Polish space ''X'' is '''analytic''' if it is the continuous image of a Borel subset of some other Polish space. Although any continuous preimage of a Borel set is Borel, not all analytic sets are Borel sets. A set is '''coanalytic''' if its complement is analytic.
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