Definition:Power Set/Class Theory

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Definition

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

$\powerset x := \set {y: y \subseteq x}$


It is clear from the definition that:

$y \in \powerset x \iff y \subseteq x$


Axiom of Powers

The concept of the power set is axiomatised in the Axiom of Powers in class theory:


Let $x$ be a set.

Then its power set $\powerset x$ is also a set.


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 2 Elements

Let $S = \set {1, 2}$.

Then the power set of $S$ is:

$\powerset S = \set {\O, \set 1, \set 2, \set {1, 2} }$

and so has $2^2 = 4$ elements.


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 the power set can be found here.


Sources

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