User:Blacklemon67/Gδ-and-Fσ-sets

In the mathematical field of topology, G_δ and F_σ sets are subsets of a topological space that generalize the concepts of open and closed sets. Accordingly, G_δ and F_σ sets are dual.

These sets are the second level of the Borel hierarchy.

History

The notation for G_δ sets originated in Germany with G for Gebiet (German: area, or neighborhood) meaning open set in this case and δ for Durchschnitt (German: intersection). The notation for F_σ sets originated in France with F for fermé (French: closed) and σ for somme (French: sum, union).

Definition

In a topological space a G_δ set is a countable intersection of open sets. The G_δ sets are the same as $\mathbf {\Pi } _{2}^{0}$ sets of the Borel hierarchy.

An F_σ is a countable union of closed sets. The F_σ sets are the same as $\mathbf {\Sigma } _{2}^{0}$ in the Borel hierarchy.

Examples

• Any open set is trivially a G_δ set. Likewise, any closed set is an F_σ set.

• The irrational numbers are a G_δ set in R, the real numbers, as they can be written as the intersection over all rational numbers q of the complement of {q} in R. The irrationals are not a F_σ set.

• The set of rational numbers Q is a F_σ set. It is not a G_δ set in R. If we were able to write Q as the intersection of open sets A_n, each A_n would have to be dense in R since Q is dense in R. However, the construction above gave the irrational numbers as a countable intersection of open dense subsets. Taking the intersection of both of these sets gives the empty set as a countable intersection of open dense sets in R, a violation of the Baire category theorem.

• In a Tychonoff space, each countable set is an F_σ set, because a point ${x}$ is closed.

  For example, the set $A$ of all points $(x,y)$ in the Cartesian plane such that $x/y$ is rational is an F_σ set because it can be expressed as the union of all the lines passing through the origin with rational slope:

  $A=\bigcup _{r\in \mathbb {Q} }\{(ry,y)\mid y\in \mathbb {R} \},$

  where $\mathbb {Q}$, is the set of rational numbers, which is a countable set.

• The zero-set of a derivative of an everywhere differentiable real-valued function on R is a G_δ set; it can be a dense set with empty interior, as shown by Pompeiu's construction.

A more elaborate example of a G_δ set is given by the following theorem:

Theorem: The set $D=\left\{f\in C([0,1]):f{\text{ is not differentiable at any point of }}[0,1]\right\}$ contains a dense G_δ subset of the metric space $C([0,1])$. (See Weierstrass function#Density of nowhere-differentiable functions.)

Properties

Basic properties

• The complement of a G_δ set is an F_σ set and vice-versa.

• The intersection of countably many G_δ sets is a G_δ set, and the union of finitely many G_δ sets is a G_δ set; a countable union of G_δ sets is called a G_δσ set.

• The union of countably many F_σ sets is an F_σ set, and the intersection of finitely many F_σ sets is an F_σ set.

• In metrizable spaces, every closed set is a G_δ set and, dually, every open set is an F_σ set.

• A subspace A of a completely metrizable space X is itself completely metrizable if and only if A is a G_δ set in X.

The following results regard Polish spaces:

• Let $({\mathcal {X}},{\mathcal {T}})$ be a Polish topological space and let $G\subset {\mathcal {X}}$ be a G_δ set (with respect to ${\mathcal {T}}$). Then $G$ is a Polish space with respect to the subspace topology on it.

• Topological characterization of Polish spaces: If ${\mathcal {X}}$ is a Polish space then it is homeomorphic to a G_δ subset of a compact metric space.

Properties of G_δ sets

The notion of G_δ sets in metric (and topological) spaces is strongly related to the notion of completeness of the metric space as well as to the Baire category theorem. This is described by the Mazurkiewicz theorem:

Theorem (Mazurkiewicz): Let $({\mathcal {X}},\rho )$ be a complete metric space and $A\subset {\mathcal {X}}$. Then the following are equivalent:

1. $A$ is a G_δ subset of ${\mathcal {X}}$
2. There is a metric $\sigma$ on $A$ which is equivalent to $\rho |A$ such that $(A,\sigma )$ is a complete metric space.

A key property of $G_{\delta }$ sets is that they are the possible sets at which a function from a topological space to a metric space is continuous. Formally: The set of points where a function $f$ is continuous is a $G_{\delta }$ set. This is because continuity at a point $p$ can be defined by a $\Pi _{2}^{0}$ formula, namely: For all positive integers $n$, there is an open set $U$ containing $p$ such that $d(f(x),f(y))<1/n$ for all $x,y$ in $U$. If a value of $n$ is fixed, the set of $p$ for which there is such a corresponding open $U$ is itself an open set (being a union of open sets), and the universal quantifier on $n$ corresponds to the (countable) intersection of these sets. In the real line, the converse holds as well; for any G_δ subset A of the real line, there is a function f: R → R which is continuous exactly at the points in A. 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 which is continuous only on the rational numbers.

G_δ space

A G_δ space is a topological space in which every closed set is a G_δ set (Johnson 1970). A normal space which is also a G_δ space is perfectly normal. Every metrizable space is perfectly normal, and every perfectly normal space is completely normal: neither implication is reversible.

See also

• Borel hierarchy
• P-space, any space having the property that every G_δ set is open and every F_σ is closed.

References

• John L. Kelley, General topology, van Nostrand, 1955. P.134.
• Steen, Lynn Arthur; Seebach, J. Arthur Jr. (1995) [1978]. Counterexamples in Topology (Dover reprint of 1978 ed.). Berlin, New York: Springer-Verlag. ISBN 978-0-486-68735-3. MR 0507446. P. 162.
• Fremlin, D.H. (2003) [2003]. "4, General Topology". Measure Theory, Volume 4. Petersburg, England: Digital Books Logostics. ISBN 0-9538129-4-4. Retrieved 1 April 2011. P. 334.
• Roy A. Johnson (1970). "A Compact Non-Metrizable Space Such That Every Closed Subset is a G-Delta". The American Mathematical Monthly, Vol. 77, No. 2, pp. 172–176. on JStor

Category:General topology Category:Descriptive set theory