 F-sigma Set
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F-sigma Set

In mathematics, an F? set (said F-sigma set) is a countable union of closed sets. The notation originated in French with F for fermé (French: closed) and ? for somme (French: sum, union).

In metrizable spaces, every open set is an F? set. The complement of an F? set is a Gδ set. In a metrizable space, any closed set is a G? set.

The union of countably many F? sets is an F? set, and the intersection of finitely many F? sets is an F? set. F? is the same as $\mathbf {\Sigma } _{2}^{0}$ in the Borel hierarchy.

## Examples

Each closed set is an F? set.

The set $\mathbb {Q}$ of rationals is an F? set. The set $\mathbb {R} \setminus \mathbb {Q}$ of irrationals is not a F? set.

In a Tychonoff space, each countable set is an F? set, because a point ${x}$ is closed.

For example, the set $A$ of all points $(x,y)$ in the Cartesian plane such that $x/y$ is rational is an F? set because it can be expressed as the union of all the lines passing through the origin with rational slope:

$A=\bigcup _{r\in \mathbb {Q} }\{(ry,y)\mid y\in \mathbb {R} \},$ where $\mathbb {Q}$ , is the set of rational numbers, which is a countable set.