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). The complement of an Fσ set is a Gδ set. Fσ is the same as in the Borel hierarchy.

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). The complement of an Fσ set is a Gδ set. Fσ is the same as in the Borel hierarchy.