Cantor-Bernstein-Schröder Theorem/Lemma

Theorem
Let $S$ be a set.

Let $T \subseteq S$.

Suppose that $f: S \to T$ is an injection.

Then there is a bijection $g: S \to T$.

Proof 1
Recursively define a sequence $\langle C_n \rangle$ in the power set of $S$ as follows:


 * $C_0 = S \setminus T$, the difference between $S$ and $T$.
 * $C_{n+1} = f \left[{C_n}\right]$, the image of $C_n$ under $f$.

Let $\displaystyle C := \bigcup_{n \mathop \in \N} C_n$.

Define a mapping $h: S \to T$ as follows:


 * $\displaystyle h \left({x}\right) = \begin{cases} f \left({x}\right) & : x \in C \\ x & : x \notin C \end{cases}$

$h$ is a mapping from $S$ to $T$
By Law of Excluded Middle, for each $x \in S$:


 * $x \in C$

or:
 * $x \notin C$

Thus the construction produces a mapping on S.

It remains to be shown that:
 * $\forall x \in S: h \left({x}\right) \in T$

Let $x \in C$.

Then:
 * $h \left({x}\right) = f \left({x}\right) \in T$

Let $x \notin C$.

Then by Set is Subset of Union:
 * $x \notin C_0$

As $x \in S$:
 * $x \in S \setminus C_0$

by the definition of set difference.

By Relative Complement of Relative Complement:


 * $S \setminus C_0 = T$

Thus $x \in T$ by the definition of subset.

As $h \left({x}\right) = x$ in this case:


 * $h \left({x}\right) \in T$

$h$ is injective
Let $x, y \in S$.

Suppose that $h \left({x}\right) = h \left({y}\right)$.

By Law of Excluded Middle for Two Variables:


 * $\left({x \in C}\right) \land \left({y \in C}\right)$

or:
 * $\left({x \notin C}\right) \land \left({y \notin C}\right)$

or:
 * $\left({x \in C}\right) \land \left({y \notin C}\right)$

or:
 * $\left({x \notin C}\right) \land \left({y \in C}\right)$

Let $x, y \in C$.

Then $x = y$ because $f$ is injective.

Let $x, y \notin C$.

Then $x = y$ by Identity Mapping is Injection.

Let $x \in C$ and $y \notin C$.

Then $x \in C_n$ for some $n$ by the definition of union.

By Set is Subset of Union:
 * $h \left({x}\right) = f \left({x}\right) \in C_{n+1} \subseteq C$

Thus $h \left({x}\right) \in C$.


 * $h \left({y}\right) = y$ by the definition of $h$.

Since $y \notin C$, this contradicts the assumption that $h \left({x}\right) = h \left({y}\right)$.

The argument for the case of $x \notin C$ and $y \in C$ is identical to the preceding.

Thus $h \left({x}\right) = h \left({y}\right) \implies x = y$ for all $x, y \in S$, so $h$ is injective.

$h$ is surjective
Let $y \in T$.

By Law of Excluded Middle:
 * $y \in C$

or:
 * $y \notin C$

Let $y \notin C$.

Then:
 * $h \left({y}\right) = y$

Let $y \in C$.

Then as $y \notin C_0 = S \setminus T$:


 * $\exists n \in \N_{>0}: y \in C_{n+1}$

Thus:
 * $\exists x \in C_n: y = f \left({x}\right)$

By the definition of union:
 * $x \in C$

Thus:
 * $h \left({x}\right) = f \left({x}\right) = y$

Thus:
 * $\forall y \in T: \exists x \in S: h \left({x}\right) = y$

Thus by definition, $h$ is surjective.

Proof 2
Define a mapping $E: \mathcal P \left({S}\right) \to \mathcal P \left({S}\right)$ as:


 * $E \left({K}\right) = S \setminus \left({T \setminus f \left({K}\right)}\right)$

Then:
 * $E \left({K}\right) = \left({S \setminus T}\right) \cup f \left({K}\right)$

By the corollary to Set Union Preserves Subsets, $E$ is increasing.

Thus by Knaster-Tarski Lemma/Power Set, $E$ has a fixed point $X$.

Then by the definition of fixed point:
 * $E \left({X}\right) = X$

That is:
 * $\left({S \setminus \left({T \setminus f \left({X}\right)}\right)}\right) = X$

Taking the set difference from $S$:


 * $T \setminus f \left({X}\right) = S \setminus X$

Let $f'$ be the restriction of $f$ to $X \times f \left({X}\right)$.

By Restricting Injection to Bijection, $f'$ is a bijection.

By Identity Mapping is Bijection, the identity mapping $I_{S \mathop \setminus X}$ from $S \setminus X$ to $T \setminus f \left({X}\right)$ is a bijection.

Thus by Union of Bijections with Disjoint Domains and Codomains is Bijection, $g = f' \cup i$ is the bijection we seek.