F-sigma set
|
In mathematics, an Fσ set (said F-sigma set) is a countable union of closed sets. 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.