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