# Definition:Power Set

## Definition

The power set of a set $S$ is the set defined and denoted as:

$\powerset S := \set {T: T \subseteq S}$

That is, the set whose elements are all of the subsets of $S$.

Note that this is a set all of whose elements are themselves sets.

It is clear from the definition that:

$T \in \powerset S \iff T \subseteq S$

### Class Theory

The power set of a set $S$ is the class of all the subsets of $S$:

$\powerset S := \set {T: T \subseteq S}$

## Axiomatic Set Theory

The concept of the power set is axiomatised in the Axiom of Powers in Zermelo-Fraenkel set theory:

For every set, there exists a set of sets whose elements are all the subsets of the given set.

$\forall x: \exists y: \paren {\forall z: \paren {z \in y \iff \paren {w \in z \implies w \in x} } }$

## Also known as

The rendition powerset is frequently seen.

Some sources do not use the term power set, merely referring to the term set of all subsets.

Variants of $\PP$ are seen throughout the literature: $\mathfrak P, P, \mathscr P, \mathrm P, \mathbf P$, etc.

Some sources, for example J.A. Green: Sets and Groups, use $\mathscr B$.

Another significant notation is:

$2^S := \set {T: T \subseteq S}$

This is used by, for example, Allan Clark: Elements of Abstract Algebra.

The relevance of this latter notation is clear from the fact that if $S$ has $n$ elements, then $2^S$ has $2^n$ elements‎.

## Examples

### Set of 3 Elements

Let $S = \set {a, b, c}$.

Then the power set of $S$ is:

$\powerset S = \set {\O, \set a, \set b, \set c, \set {a, b}, \set {b, c}, \set {a, c}, S}$

and so has $2^3 = 8$ elements.

Note that while $\set a \in \powerset S$, $a \notin \powerset S$.

## Also see

• Results about power sets can be found here.