# Cantor-Schröder-Bernstein Theorem

The Cantor-Schröder-Bernstein theorem $$\left( {\text{CSB}} \right)$$ states that for any two sets $$A$$ and $$B,$$ if $$\left| A \right| \le \left| B \right|$$ and $$\left| B \right| \le \left| A \right|,$$ then $$\left| A \right| = \left| B \right|.$$

In terms of functions, the Cantor-Schröder-Bernstein theorem states that if $$A$$ and $$B$$ are sets and there are injective functions $$f : A \to B$$ and $$g : B \to A,$$ then there exists a bijective function $$h : A \to B.$$

In terms of relation properties, the Cantor-Schröder-Bernstein theorem shows that the order relation on cardinalities of sets is antisymmetric.

$${\text{CSB}}$$ is a fundamental theorem of set theory. It is a convenient tool for comparing cardinalities of infinite sets.

## Proof

There are many different proofs of this theorem. We present here a direct proof by using the definitions of injective and surjective function.

Let $$A, B$$ be sets and let $$f: A \to B$$ and $$g : B \to A$$ be injective functions. We need to show that there is a bijective function $$h : A \to B.$$

We will denote the range of the function $$f$$ by $$f\left( A \right)$$ and the range of the function $$g$$ by $$g\left( B \right).$$ By condition, the functions $$f: A \to B$$ and $$g : B \to A$$ are injective, so they may not have inverse functions (unless they are also surjective). However, the function $$g : B \to g\left( B \right)$$ between sets $$B$$ and $$g\left( B \right)$$ is bijective, and therefore it has an inverse $$g^{-1} : g\left( B \right) \to B.$$

Successively applying the injections $$g$$ and $$f,$$ we get the following fractal-like structures:

Let $${A_0} = A\backslash g\left( B \right)$$ be the complement of $$g\left( B \right)$$ after the function $$g$$ is applied for the first time. When we circle back to $$A$$ with the next iteration, we get $${A_1} = \left( {g \circ f} \right)\left( {{A_0}} \right).$$ Each cycle creates a new nested region $$A_1, A_2, \ldots, A_n, A_{n+1},\ldots$$ (shaded in blue) according to the recurrence relation

${A_{n + 1}} = \left( {g \circ f} \right)\left( {{A_n}} \right).$

We will use the symbol $${A_\infty }$$ to denote the union over all $$n,$$ so the entire blue region of the set $$A$$ is given by

${A_\infty } = \bigcup\limits_{n = 0}^\infty {{A_n}} = \bigcup\limits_{n = 0}^\infty {{{\left( {g \circ f} \right)}^n}\left( {A\backslash g\left( B \right)} \right)} .$

The remaining (orange) region in $$A$$ is expressed as $$A \backslash A_{\infty}.$$

Thus, we partitioned the original set $$A$$ into two subsets that "behave" differently. The first subset $$A_{\infty}$$ maps the blue areas in $$A$$ to the blue areas in $$B$$ under the function $$f.$$ The second subset $$A \backslash A_{\infty}$$ maps the orange areas in $$A$$ to the orange areas in $$B$$ using the function $$g^{-1}.$$

The resulting function $$h : A \to B$$ is defined as follows:

$h\left( x \right) = \left\{ {\begin{array}{*{20}{l}} {f\left( x \right)} &{\text{if }x \in {A_\infty }}\\ {{g^{ - 1}}\left( x \right)} &{\text{otherwise}} \end{array}} \right..$

To see that $$h$$ is injective, we pick two elements $$x,y \in A$$ and show that $$h\left( x \right) = h\left( y \right)$$ implies $$x = y.$$ Consider three cases:

1. If $$x,y \in A_{\infty},$$ then
$h\left( x \right) = h\left( y \right),\;\; \Rightarrow f\left( x \right) = f\left( y \right),\;\; \Rightarrow x = y,$
because the function $$f$$ is injective.
2. If $$x,y \not\in A_{\infty},$$ then given that $$g$$ is injective, we get
$h\left( x \right) = h\left( y \right),\;\; \Rightarrow {g^{ - 1}}\left( x \right) = {g^{ - 1}}\left( y \right),\;\; \Rightarrow \left( {g \circ {g^{ - 1}}} \right)\left( x \right) = \left( {g \circ {g^{ - 1}}} \right)\left( y \right),\;\; \Rightarrow x = y.$
3. If $$x \in A_{\infty}$$ but $$y \not\in A_{\infty},$$ then $$x \in A_n$$ for some $$n.$$ This yields:
$h\left( x \right) = h\left( y \right),\;\; \Rightarrow f\left( x \right) = {g^{ - 1}}\left( y \right),\;\; \Rightarrow \left( {g \circ f} \right)\left( x \right) = \left( {g \circ {g^{ - 1}}} \right)\left( y \right),\;\; \Rightarrow y = \left( {g \circ f} \right)\left( x \right) \in \left( {g \circ f} \right)\left( {{A_n}} \right) = {A_{n + 1}}.$
This contradicts the assumption that $$y \not\in A_{\infty}.$$ Hence, this case cannot happen.

To see that $$h$$ is surjective, we take an arbitrary element $$b \in B$$ and show that there is a preimage $$x \in A$$ such that $$h\left( x \right) = b.$$ There are two cases to consider:

1. If $$x = g\left( b \right) \not\in A_{\infty},$$ then
$h\left( x \right) = {g^{ - 1}}\left( x \right) = {g^{ - 1}}\left( {g\left( b \right)} \right) = \left( {{g^{ - 1}} \circ g} \right)\left( b \right) = b.$
2. If $$x = g\left( b \right) \in A_{\infty},$$ then $$x \in A_n$$ for some $$n.$$ Here $$n \gt 0$$ because $$x \in g\left( B \right).$$ Using the recurrence relation $${A_{n}} = \left( {g \circ f} \right)\left( {{A_{n-1}}} \right),$$ we can write $$x \in \left( {g \circ f} \right)\left( {{A_{n-1}}} \right).$$ If we take an element $$z \in A_{n-1},$$ then we have $$x = \left( {g \circ f} \right)\left( z \right) = g\left( {f\left( z \right)} \right).$$ The function $$g$$ is injective. Therefore
$\left\{ \begin{array}{l} x = g\left( b \right)\\ x = g\left( {f\left( z \right)} \right) \end{array} \right., \Rightarrow b = f\left( z \right).$
We can choose $$x = z$$ to have
$h\left( x \right) = f\left( x \right) = f\left( z \right) = b.$

Thus for any $$b \in B,$$ there is an element $$x \in A$$ for which $$h\left( x \right) = b.$$ So, the function $$h$$ is surjective. Since $$h$$ is both injective and surjective, it is bijective.

## Corollary 1. $$\left| {\left( {0,1} \right]} \right| = \left| {\left( {0,1} \right)} \right|$$

We have already found a bijective function between the sets $${\left( {0,1} \right]}$$ and $${\left( {0,1} \right)}$$ in Example $$3$$ on the Cardinality of a Set page. Now we solve the problem by using the Cantor-Schröder-Bernstein theorem.

The function $$f\left( x \right) = \frac{x}{2} + \frac{1}{4}$$ is an injection $$\left( {0,1} \right] \to \left( {0,1} \right).$$ Also, the function $$g\left( x \right) = x$$ is an injection $$\left( {0,1} \right) \to \left( {0,1} \right].$$ It then follows from the CSB theorem that

$\left| {\left( {0,1} \right]} \right| = \left| {\left( {0,1} \right)} \right|.$

Thus, in order to prove that two sets have the same cardinality, we can just look for injections both ways instead of looking for a bijection.

## Corollary 2. Equal Cardinalities of Unit Square and Unit Interval

Consider the open unit square $$I \times I = \left( {0,1} \right) \times \left( {0,1} \right)$$ and the open unit interval $$I = \left( {0,1} \right).$$

To build an injection from $$I \times I$$ to $$I,$$ we represent the coordinates $$\left( {x,y} \right)$$ of an arbitrary point of the square by their decimal expansions. Let

$x = 0.{x_1}{x_2}{x_3} \ldots ,\;\;y = 0.{y_1}{y_2}{y_3} \ldots$

We assume here that $$x$$ and $$y$$ do not have a repeating sequence of $$9'\text{s}$$ at the end. If there exists such a number, it must be represented as a terminating decimal:

$0.73699999 \ldots = 0.737$

We define the mapping $$f:I \times I \to I$$ as follows:

$f\left( {\left( {x,y} \right)} \right) = z = 0.{x_1}{y_1}{x_2}{y_2}{x_3}{y_3} \ldots$

If $$\left( {{x},{y}} \right) \ne \left( {{x^\prime},{y^\prime}} \right),$$ then the two numbers have a different coordinate, and hence, their decimal expansions are also different, that is, $${z_1} \ne {z_2}.$$ This means that the function $$f$$ is injective.

There is a natural injection from $$I$$ to $$I \times I:$$ $$g\left( z \right) = \left( {a,z} \right),$$ where $$a$$ is any number from the interval $$\left( {0,1} \right).$$ For example, if we take $$a = \frac{1}{2},$$ then $$g\left( z \right) = \left( {\frac{1}{2}, z} \right).$$

By the Cantor-Schröder-Bernstein theorem,

$\left| {\left( {0,1} \right) \times \left( {0,1} \right)} \right| = \left| {\left( {0,1} \right)} \right|.$

Thus, the open unit square and the open unit interval have the same cardinality, which is an amazing result!

## Corollary 3. $$\left| \mathbb{R} \right| = \left| {\mathbb{R} \times \mathbb{R}} \right|$$

We can map $$I \to \mathbb{R}$$ using the function

$f\left( x \right) = \tan \left( {\pi x - \frac{\pi }{2}} \right).$

This mapping is bijective.

Similarly, the mapping $$I \times I \to \mathbb{R} \times \mathbb{R}$$ is given by the function

$g\left( {x,y} \right) = \left( {\tan \left( {\pi x - \frac{\pi }{2}} \right),\tan \left( {\pi y - \frac{\pi }{2}} \right)} \right)$

that is also bijective.

Then we have

$\left| {\mathbb{R} \times \mathbb{R}} \right| = \left| {\left( {0,1} \right) \times \left( {0,1} \right)} \right| = \left| {\left( {0,1} \right)} \right| = \left| \mathbb{R} \right|,$

that is, the set of points of a plane and the set of points of a number line have the same cardinality.

## Corollary 4. $$\left| \mathbb{R} \right| = \left| {\mathcal{P}\left( \mathbb{N} \right)} \right|$$

Notice that the cardinality of $$\mathbb{R}$$ is the same as the cardinality of the open unit interval $$I = \left( {0,1} \right)$$ because there exists a bijective function $$f: I \to \mathbb{R}$$ between the sets:

$f\left( x \right) = \tan \left( {\pi x - \frac{\pi }{2}} \right).$

Next, similarly to the Corollary $$1,$$ we can easily prove that the open interval $${\left( {0,1} \right)}$$ and the closed interval $${\left[ {0,1} \right]}$$ have the same cardinality. Hence, we will just need to show that $$\left| {\left[ {0,1} \right]} \right| = \left| {\mathcal{P}\left( \mathbb{N} \right)} \right|.$$ By the Cantor-Schröder-Bernstein theorem, it suffices to find injections $$\left[ {0,1} \right] \to \mathcal{P}\left( \mathbb{N} \right)$$ and $$\mathcal{P}\left( \mathbb{N} \right) \to \left[ {0,1} \right].$$

To define $$\left[ {0,1} \right] \to \mathcal{P}\left( \mathbb{N} \right),$$ we use the fact that any real can be represented as a binary decimal. Let

$x = 0.{b_1}{b_2}{b_3}{b_4} \ldots \in \left[ {0,1} \right], \;\text{where }\;{b_i} \in \left\{ {0,1} \right\}.$

Then the mapping function is given by

$f\left( x \right) = \left\{ {i \in \mathbb{N} \mid {b_i} = 1} \right\}.$

For example,

$f\left( {0.110001 \cdots } \right) = \left\{ {1,2,6, \ldots } \right\},\;\;f\left( {0.00111010 \cdots } \right) = \left\{ {3,4,5,7, \ldots } \right\}.$

The function $$f$$ is injective.

To define $$\mathcal{P}\left( \mathbb{N} \right) \to \left[ {0,1} \right],$$ we use the similar approach. Let $$S$$ be a subset of $$\mathbb{N}.$$ The function $$g\left( S \right)$$ is given by

$g\left( S \right) = 0.{b_1}{b_2}{b_3}{b_4} \ldots ,$

where $$b_i = 1$$ if $$i \in S$$ and $$b_i = 0$$ if $$i \not\in S.$$ For example,

$g\left( {\left\{ {1,2,7,8, \ldots } \right\}} \right) = 0,11000011 \cdots,\;\;g\left( {\left\{ {4,5,6, \ldots } \right\}} \right) = 0,000111 \cdots .$

We also set

$g\left( \varnothing \right) = 0,\;\;g\left( N \right) = 0.11111 \cdots .$

It is clear that the function $$g$$ is injective.

By the Cantor-Schröder-Bernstein theorem, $$\left| \left[ {0,1} \right]\right| = \left| {\mathcal{P}\left( \mathbb{N} \right)} \right|$$ and hence

$\left| \mathbb{R} \right| = \left| {\mathcal{P}\left( \mathbb{N} \right)} \right|.$

Note that if $$A$$ is an arbitrary set, its power set $$\mathcal{P}\left( A \right)$$ can be represented as

$\left| {P\left( A \right)} \right| = {2^{\left| A \right|}}.$

Indeed, with every subset $$B \subseteq A,$$ we can associate the characteristic function $${\chi _B}:A \to \left\{ {0,1} \right\}$$ defined by

${\chi _B}\left( a \right) = \left\{ {\begin{array}{*{20}{l}} {1} &{\text{if}\;\;a \in B}\\ {0} &{\text{if}\;\;a \not\in B} \end{array}} \right.,$

where $$a \in A.$$

Clearly, the total number of subsets of $$A$$ is $$2^{\left| A \right|}.$$ For the set of natural numbers, we obtain

$\left| {\mathcal{P}\left( \mathbb{N} \right)} \right| = {2^{\left| \mathbb{N} \right|}}.$

Then

$\left| \mathbb{R} \right| = \left| {\mathcal{P}\left( \mathbb{N} \right)} \right| = {2^{\left| \mathbb{N} \right|}}.$

See solved problems on Page 2.