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In mathematics, an Fσ set is a countable union of closed sets. The notation originated in French with F for fermé and σ for somme.
The complement of an Fσ set is a Gδ set.
Fσ is the same as Σ 2 0 {\displaystyle \mathbf {\Sigma } _{2}^{0}} in the Borel hierarchy.
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