Axiom of power set

In mathematics, the axiom of power set is one of the Zermelo-Fraenkel axioms of axiomatic set theory.

In the formal language of the Zermelo-Fraenkel axioms, the axiom reads:

Failed to parse (Missing texvc executable; please see math/README to configure.): \forall A, \exists\; {\mathcal{P}(A)}, \forall B: B \in {\mathcal{P}(A)} \iff (\forall C: C \in B \implies C \in A).

Or in other words:

Given any set A, there is a set Failed to parse (Missing texvc executable; please see math/README to configure.): \mathcal{P}(A)

such that, given any set B, B is a member of Failed to parse (Missing texvc executable; please see math/README to configure.): \mathcal{P}(A)
if and only if B is a subset of A.  (Subset is not used in the formal definition above because the axiom of power set is an axiom that may need to be stated without reference to the concept of subset.)

By the axiom of extensionality this set is unique. We call the set Failed to parse (Missing texvc executable; please see math/README to configure.): \mathcal{P}(A)

the power set of A. Thus, the essence of the axiom is that every set has a power set.

The axiom of power set is generally considered uncontroversial and it, or an equivalent axiom, appears in most alternative axiomatizations of set theory.

Consequences

The Power Set Axiom allows the definition of the Cartesian product of two sets Failed to parse (Missing texvc executable; please see math/README to configure.): X

and Failed to parse (Missing texvc executable; please see math/README to configure.): Y

Failed to parse (Missing texvc executable; please see math/README to configure.): X \times Y = \{ (x, y) : x \in X \land y \in Y \}.

The Cartesian product is a set since

Failed to parse (Missing texvc executable; please see math/README to configure.): X \times Y \subseteq \mathcal{P}(\mathcal{P}(X \cup Y)).

One may define the Cartesian product of any finite collection of sets recursively:

Failed to parse (Missing texvc executable; please see math/README to configure.): X_1 \times \cdots \times X_n = (X_1 \times \cdots \times X_{n-1}) \times X_n.

Note that the existence of the Cartesian product can be proved in Kripke–Platek set theory which does not contain the power set axiom.

References

• Paul Halmos, Naive set theory. Princeton, NJ: D. Van Nostrand Company, 1960. Reprinted by Springer-Verlag, New York, 1974. ISBN 0-387-90092-6 (Springer-Verlag edition).
• Jech, Thomas, 2003. Set Theory: The Third Millennium Edition, Revised and Expanded. Springer. ISBN 3-540-44085-2.
• Kunen, Kenneth, 1980. Set Theory: An Introduction to Independence Proofs. Elsevier. ISBN 0-444-86839-9.

This article incorporates material from Axiom of power set on PlanetMath, which is licensed under the GFDL.fa:اصل موضوع مجموعه توانی fr:Axiome de l'ensemble des parties it:Assioma dell'insieme potenza lmo:Assioma dal cungjuunt da le parte pt:Axioma da potência sv:Potensmängdsaxiomet