In mathematics, an F_σ set (said F-sigma set) is a countable union of closed sets. In metrizable spaces, every open set is an F_σ set.

The complement of a F_σ set is a G_δ set. In a metrizable space, any closed set is a G_δ set.

The union of countably many F_σ sets is an F_σ set, and the intersection of finitely many F_σ sets is an F_σ set.