In mathematics, an Fσ set (said F-sigma set) is a countableunion of closed sets. The notation originated in French with F for fermé (French: closed) and σ for somme (French: sum, union).[1]
The union of countably many Fσ sets is an Fσ set, and the intersection of finitely many Fσ sets is an Fσ set.
The set of all points in the Cartesian plane such that 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.