Definition:F-Sigma Set

Definition
Let $\left({X, \vartheta}\right)$ 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 $X$.

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 union.

Also see

 * $G_\delta$ (G-Delta) Set


 * Complement of F-Sigma Set is G-Delta Set