Lusin's separation theorem
In descriptive set theory and mathematical logic, Lusin's separation theorem states that if A and B are disjoint analytic subsets of Polish space, then there is a Borel set C in the space such that A ⊆ C and B ∩ C = ∅. It is named after Nikolai Luzin, who proved it in 1927.
The theorem can be generalized to show that for each sequence (An) of disjoint analytic sets there is a sequence (Bn) of disjoint Borel sets such that An ⊆ Bn for each n. 
An immediate consequence is Suslin's theorem, which states that if a set and its complement are both analytic, then the set is Borel.
- Kechris, Alexander (1995), Classical descriptive set theory, Graduate texts in mathematics 156, Berlin–Heidelberg–New York: Springer–Verlag, pp. xviii+402, doi:10.1007/978-1-4612-4190-4, ISBN 0-387-94374-9, MR 1321597, Zbl 0819.04002 (ISBN 3-540-94374-9 for the European edition)
- Lusin, Nicolas (1927), "Sur les ensembles analytiques", Fundamenta Mathematicae (in French) 10: 1–95, JFM 53.0171.05.
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