In the mathematical field of topology, a Gδ set is a subset of a topological space that is a countable intersection of open sets.
The notation originated from the German nouns Gebiet 'open set' and Durchschnitt 'intersection'.
[1] Historically Gδ sets were also called inner limiting sets,[2] but that terminology is not in use anymore.
Gδ sets, and their dual, F𝜎 sets, are the second level of the Borel hierarchy.
In a topological space a Gδ set is a countable intersection of open sets.
The notion of Gδ sets in metric (and topological) spaces is related to the notion of completeness of the metric space as well as to the Baire category theorem.
See the result about completely metrizable spaces in the list of properties below.
sets and their complements are also of importance in real analysis, especially measure theory.
corresponds to the (countable) intersection of these sets.
As a consequence, while it is possible for the irrationals to be the set of continuity points of a function (see the popcorn function), it is impossible to construct a function that is continuous only on the rational numbers.
In the real line, the converse holds as well; for any Gδ subset
[9] A Gδ space[10] is a topological space in which every closed set is a Gδ set.
For example, every metrizable space is perfectly normal.