# axiom of power set

The axiom of power set  is an axiom of Zermelo-Fraenkel set theory  which postulates  that for any set $X$ there exists a set $\mathcal{P}(X)$, called the power set  of $X$, consisting of all subsets of $X$. In symbols, it reads:

 $\forall X\exists\mathcal{P}(X)\forall u(u\in\mathcal{P}(X)\leftrightarrow u% \subseteq X).$

In the above, $u\subseteq X$ is defined as $\forall z(z\in u\rightarrow z\in X)$. By the extensionality axiom, the set $\mathcal{P}(X)$ is unique.

The Power Set Axiom allows us to define the Cartesian product  of two sets $X$ and $Y$:

 $X\times Y=\{(x,y):x\in X\land y\in Y\}.$

The Cartesian product is a set since

 $X\times Y\subseteq\mathcal{P}(\mathcal{P}(X\cup Y)).$

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

 $X_{1}\times\cdots\times X_{n}=(X_{1}\times\cdots\times X_{n-1})\times X_{n}.$
Title axiom of power set AxiomOfPowerSet 2013-03-22 13:43:03 2013-03-22 13:43:03 mathcam (2727) mathcam (2727) 11 mathcam (2727) Axiom msc 03E30 power set axiom powerset axiom axiom of powerset