TOPICS
Search

G_delta Set


A subset G subset R of the real numbers is said to be a G_delta set provided G is the countable intersection of open sets. The name G_delta comes from German: The G stands for Gebiet, meaning "area," while the delta stands for Durchschnitt, meaning "intersection."

This definition is dual to that of an Fsigma set in the sense that G is a G_delta set if and only if its complement G^c=R-G is an F_sigma.

Note that both G_delta and F_sigma sets are fundamental to measure theory. In particular, both types of sets are Borel sets and are considered to be the second level of the so-called Borel hierarchy.


See also

Borel Hierarchy, Borel Set, Closed Set, Fsigma Set, Open Set

This entry contributed by Christopher Stover

Explore with Wolfram|Alpha

References

Royden, H. L. and Fitzpatrick, P. M. Real Analysis. Pearson, 2010.

Cite this as:

Stover, Christopher. "G_delta Set." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/G-DeltaSet.html

Subject classifications