# Definition:F-Sigma Set

## Definition

Let $T = \left({S, \tau}\right)$ be a topological space.

An $F_\sigma$ set (F-sigma set) is a set which can be written as a countable union of closed sets of $T$.

## Also see

• Results about $F_\sigma$ sets can be found here.

## Linguistic Note

The name $F_\sigma$ originates from the French:

F for fermé, which is French for closed
$\sigma$ (Greek s) for somme, which is French for set union.