Bijection iff exists Mapping which is Left and Right Inverse

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Let $S$ and $T$ be sets.

Let $f: S \to T$ be a mapping.

Then $f$ is a bijection if and only if:

there exists a mapping $g: T \to S$ such that:
$g \circ f = I_S$
$f \circ g = I_T$
where $I_S$ and $I_T$ are the identity mappings on $S$ and $T$ respectively.


Necessary Condition

Let $f$ be a bijection.

Then for each $y \in T$ there exists one and only one $x \in S$ such that $f \left({x}\right) = y$.

That is, that there exists a mapping $g: T \to S$ with the property that:

$\forall y \in T: \exists x \in S: \map g y = x$

Let $y \in T$.

Let $x = g \map g y$.


\(\displaystyle y\) \(=\) \(\displaystyle \map f {\map g y}\) $\quad$ $\quad$
\(\displaystyle \) \(=\) \(\displaystyle \map {f \circ g} y\) $\quad$ $\quad$


\(\displaystyle x\) \(=\) \(\displaystyle \map g {\map f x}\) $\quad$ $\quad$
\(\displaystyle \) \(=\) \(\displaystyle \map {f \circ g} x\) $\quad$ $\quad$

demonstrating that $g$ has the property that:

$g \circ f = I_S$
$f \circ g = I_T$


Sufficient Condition

Suppose there exists a $g$ which satisfies the conditions on $f$.

By Condition for Composite Mapping to be Identity:

$(1): \quad$ from $g \circ f = I_S$ it follows that $f$ is an injection.
$(2): \quad$ from $f \circ g = I_T$ it follows that $f$ is a surjection.

Thus, by definition, $f$ is a bijection.