Main Page | See live article | Alphabetical index In topology, X, a G-delta set (or Gδ set) is a countable intersection of open sets. In metrizable spaces, every closed set is a Gδ set. The complement of a Gδ set is an Fσ. In a metrizable space, every open set is an Fσ set. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.