Definition:G-Delta Set

Definition
Let $$\left({X, \vartheta}\right)$$ be a topological space.

An $$G_\delta$$ set (G-Delta set) is a set which can be written as a countable intersection of open sets of $$X$$.

Linguistic Note
The name $$G_\delta$$ originates from the German:
 * G for Gebiet, which is German for area or neighborhood (here used in the sense open set in this case;
 * $$\delta$$ (Greek d) for Durchschnitt, which is German for intersection.

Also see

 * $F_\sigma$ (F-Sigma) Set


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