# G-delta set

In

topology,

*X*, a

**G-delta set** (or

**G**_{δ} set) is a

countable intersection of open sets. In

metrizable spaces, every

closed set is a G

_{δ} set.

The complement of a G_{δ} set is an F_{σ}. In a metrizable space, every open set is an F_{σ} set.