# Cardinality of Power Set of Finite Set

## Theorem

Let $S$ be a set such that:

$\card S = n$

where $\card S$ denotes the cardinality of $S$,

Then:

$\card {\powerset S} = 2^n$

where $\powerset S$ denotes the power set of $S$.

Thus it can be seen that the power set's alternative notation $2^S$ is indeed appropriate.

However, because of possible confusion over the conventional meaning of $2^n$, its use is deprecated.

## Proof 1

Let $T = \set {0, 1}$.

For each $A \in \powerset S$, we consider the characteristic function $\chi_A: S \to T$ defined as:

$\forall x \in S: \map {\chi_A} x = \begin{cases} 1 & : x \in A \\ 0 & : x \notin A \end{cases}$

Now consider the mapping $f: \powerset S \to T^S$:

$\forall A \in \powerset S: \map f A = \chi_A$

where $T^S$ is the set of all mappings from $S$ to $T$.

Also, consider the mapping $g: T^S \to \powerset S$:

$\forall \phi \in T^S: \map g \phi = \phi^{-1} \sqbrk {\set 1}$

where $\phi^{-1} \sqbrk {\set 1}$ is the preimage of $\set 1$ under $\phi$.

Consider the characteristic function of $\phi^{-1} \sqbrk {\set 1}$, denoted $\chi_{\phi^{-1} \sqbrk {\set 1} }$.

We have:

 $\ds \forall x \in S: \,$ $\ds \map {\chi_{\phi^{-1} \sqbrk {\set 1} } } x$ $=$ $\ds \begin{cases} 1 & : x \in \phi^{-1} \sqbrk {\set 1} \\ 0 & : x \notin \phi^{-1} \sqbrk {\set 1} \end{cases}$ $\ds$ $=$ $\ds \begin{cases} 1 & : \map \phi x = 1 \\ 0 & : \map \phi x = 0 \end{cases}$ $\ds$ $=$ $\ds \map \phi x$

So:

 $\ds \forall \phi \in T^S: \,$ $\ds \map {\paren {f \circ g} } \phi$ $=$ $\ds \map f {\phi^{-1} } {\sqbrk {\set 1} }$ $\ds$ $=$ $\ds \chi_{\phi^{-1} \sqbrk {\set 1} }$ $\ds$ $=$ $\ds \phi$

So $f \circ g = I_{T^S}$, that is, the identity mapping on $T^S$.

So far so good. Now we consider the preimage of $\set 1$ under $\chi_A$:

$\chi_A^{-1} \sqbrk {\set 1} = A$

from the definition of the characteristic function $\chi_A$ above.

Thus:

 $\ds \forall A \in \powerset S: \,$ $\ds \map {\paren {g \circ f} } A$ $=$ $\ds \map g {\map f A}$ $\ds$ $=$ $\ds \map g {\chi_A}$ $\ds$ $=$ $\ds \chi_A^{-1} \sqbrk {\set 1}$ $\ds$ $=$ $\ds A$

So $g \circ f = I_{\powerset S}$, that is, the identity mapping on $\powerset S$.

It follows from Bijection iff Left and Right Inverse that $f$ and $g$ are bijections.

Thus by Cardinality of Set of All Mappings the result follows.

$\blacksquare$

## Proof 2

Enumerating the subsets of $S$ is equivalent to counting all of the ways of selecting $k$ out of the $n$ elements of $S$ with $k = 0, 1, \ldots, n$.

So, from Cardinality of Set of Subsets, the number we are looking for is:

$\ds \card {\powerset S} = \sum_{k \mathop = 0}^n \binom n k$

But from the binomial theorem:

$\ds \paren {x + y}^n = \sum_{k \mathop = 0}^n \binom n k x^{n - k} y^k$

It follows that:

$2^n = \ds \paren {1 + 1}^n = \sum_{k \mathop = 0}^n \binom n k \paren 1^{n - k} \paren 1^k = \sum_{k \mathop = 0}^n \binom n k = \card {\powerset S}$

$\blacksquare$

## Proof 3

Proof by induction:

For all $n \in \N$, let $\map P n$ be the proposition:

$\card S = n \implies \card {\powerset S} = 2^n$

Do not confuse $\map P n$, which is a propositional function on $\N$, with $\powerset S$, the power set of $S$.

### Basis for the Induction

$S = \O \iff \card S = 0$

Then:

$\powerset \O = \set \O$

has one element, that is, $\O$.

So:

$\card {\powerset \O} = \card {\set \O} = 1 = 2^0$

thus confirming that $\map P 0$ holds.

This is our basis for the induction.

### Induction Hypothesis

Now we need to show that, if $\map P k$ is true, where $k \ge 2$, then it logically follows that $\map P {k + 1}$ is true.

So this is our induction hypothesis:

$\card S = k \implies \card {\powerset S} = 2^k$

Then we need to show:

$\card S = k + 1 \implies \card {\powerset S} = 2^{k + 1}$

### Induction Step

This is our induction step:

Let $\card S = k + 1$.

Let $x \in S$.

Consider the set $S' = S \setminus \set x$ where $x$ is any element of $S$.

We see that $\card {S'} = k$.

Now adjoin $x$ to all the subsets of $S'$.

Counting the subsets of $S$, we have:

all the subsets of $S'$

and:

all the subsets of $S'$ with $x$ adjoined to them.

From the induction hypothesis, there are $2^k$ subsets of $S'$.

Adding $x$ to each of these does not change their number, so there are another $2^k$ subsets of $S$ consisting of all the subsets of $S'$ with $x$ adjoined to them.

In total that makes $2^k + 2^k = 2 \times 2^k = 2^{k + 1}$ subsets of $S$.

So $\map P k \implies \map P {k + 1}$ and the result follows by the Principle of Mathematical Induction.

Therefore:

$\forall n \in \N: \card S = n \implies \card {\powerset S} = 2^n$

$\blacksquare$

## Informal Proof

Given an element $x$ of $S$, each subset of $S$ either includes $x$ or does not include $x$ (this follows directly from the definition of a set), which gives us two possibilities.

The same reasoning holds for any element of $S$.

One can intuitively see that this means that there are $\ds \underbrace {2 \times 2 \times \ldots \times 2}_{\card S} = 2^{\card S}$ total possible combinations of elements of $S$.

This is exactly $\card {\powerset S}$.

$\blacksquare$