# 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 ${\displaystyle \mathbf {\Sigma } _{2}^{0}}$ in the Borel hierarchy.

## Examples

Each closed set is an Fσ set.

The set ${\displaystyle \mathbb {Q} }$  of rationals is an Fσ set. Furthermore any countable set in a T1 space, is an Fσ set, because a singleton set ${\displaystyle \{x\}}$  is closed.

The set ${\displaystyle \mathbb {R} \setminus \mathbb {Q} }$  of irrationals is not a Fσ set.

In metrizable spaces, every open set is an Fσ set.[2]

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 ${\displaystyle A}$  of all points ${\displaystyle (x,y)}$  in the Cartesian plane such that ${\displaystyle x/y}$  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:

${\displaystyle A=\bigcup _{r\in \mathbb {Q} }\{(ry,y)\mid y\in \mathbb {R} \},}$

where ${\displaystyle \mathbb {Q} }$ , is the set of rational numbers, which is a countable set.