Fσ set

A subset of a topological spaceMathworldPlanetmath is called a Fσ set if it equals the union of a countableMathworldPlanetmath collectionMathworldPlanetmath of closed setsPlanetmathPlanetmath.

The complement of a Fσ set is a Gδ set (http://planetmath.org/G_DeltaSet).

For instance, the X set of all points (x,y) in the plane such that either y=0 or x/y is rational is an Fσ set because it can be expressed as the union of a countable set of lines:

Title Fσ set
Canonical name FsigmaSet
Date of creation 2013-03-22 14:37:59
Last modified on 2013-03-22 14:37:59
Owner rspuzio (6075)
Last modified by rspuzio (6075)
Numerical id 9
Author rspuzio (6075)
Entry type Definition
Classification msc 54A05
Related topic G_DeltaSet
Related topic G_deltaSet
Related topic PavedSet
Related topic PavedSpace