F-sigma set
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In mathematics, an Fσ set (said F-sigma set) is a countable union of closed sets. The notation originated in France with F for fermé (French:closed) and σ for somme (French:union).
In metrizable spaces, every open set is an Fσ set.
The complement of a Fσ set is a Gδ set. In a metrizable space, any closed set is a Gδ set.
The union of countably many Fσ sets is an Fσ set, and the intersection of finitely many Fσ sets is an Fσ set.
For example, the set A of all points (x,y) in the Cartesian plane such that x / y is rational is an Fσ set because it can be expressed as the union of all the lines passing through the origin with rational slope:
where 
, is the set of rational numbers, which is a countable set.
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