In mathematics, an F_σ 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 F_σ set is a G_δ set.^[1]

F_σ is the same as ${\displaystyle \mathbf {\Sigma } _{2}^{0))$ in the Borel hierarchy.

Each closed set is an F_σ set.

The set ${\displaystyle \mathbb {Q} }$ of rationals is an F_σ set in ${\displaystyle \mathbb {R} }$. More generally, any countable set in a T₁ space is an F_σ set, because every singleton ${\displaystyle \{x\))$ is closed.

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

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

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.

The set ${\displaystyle A}$ of all points ${\displaystyle (x,y)}$ in the Cartesian plane such that ${\displaystyle x/y}$ is rational is an F_σ 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.

• G_δ set — the dual notion.
• Borel hierarchy
• P-space, any space having the property that every F_σ set is closed