Fσ_set
In mathematics, an Fσ set (said F-sigma set) is a countable union of closed sets. The notation originated in French with F for fermé (French: closed) and σ for somme (French: sum, union).[1]
The complement of an Fσ set is a Gδ set.[1]
Fσ is the same as in the Borel hierarchy.