Surjection iff Right Inverse/Non-Uniqueness

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Theorem

Let $S$ and $T$ be sets such that $S \ne \varnothing$.

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


A right inverse of $f$ is in general not unique.


Uniqueness occurs if and only if $f$ is a bijection.


Proof

If $f$ is not an injection then:

$\exists y \in T: \exists x_1, x_2 \in S: \map f {x_1} = y = \map f {x_2}$

Hence we have more than one choice in $\map {f^{-1} } {\set y}$ for how to map $\map g y$.

That is, $\map g y$ is not unique.


This does not happen if and only if $f$ is an injection.

Hence the result.

$\blacksquare$


Sources