# Injection from Finite Set to Itself is Surjection

## Theorem

Let $S$ be a finite set.

Let $f: S \to S$ be an injection.

Then $f$ is also a surjection.

### Corollary

Let $S$ be a finite set.

Let $f: S \to S$ be an injection.

Then $f$ is a permutation.

## Proof

Let $a \in S$.

We need to show that there exists $b \in S$ such that $a = \map f b$.

Consider what happens when $f$ is applied repeatedly on $S$.

Let $f^2$ denote $f \circ f$ and, generally, $f^n := f \circ f^{n-1}$.

Consider the sequence of elements of $S$:

$a, \map f a, \map {f^2} a, \ldots$

Because $S$ is a finite set, there must be repetitions.

That is, there must exist $r, s \in \N$ such that:

$\map {f^r} a = \map {f^s} a$

where $r \ne s$.

Without loss of generality, assume $r > s$.

$f$ is an injection.

Therefore by Composite of Injections is Injection, $f^s$ is an injection.

By Injection iff Left Cancellable, $f^s$ is left cancellable.

Thus:

 $\ds \map {f^r} a$ $=$ $\ds \map {f^s} a$ $\ds \leadsto \ \$ $\ds \map {f^s \circ f^{r - s} } a$ $=$ $\ds \map {f^s} a$ $\ds \leadsto \ \$ $\ds \map {f^{r - s} } a$ $=$ $\ds a$ Definition of Left Cancellable Mapping

That is, $b = \map {f^{r - s - 1} } a$.

$\blacksquare$