Subset equals Preimage of Image implies Injection

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

Let $f^\to: \mathcal P \left({S}\right) \to \mathcal P \left({T}\right)$ be the mapping induced by $f$.

Similarly, let $f^\gets: \mathcal P \left({T}\right) \to \mathcal P \left({S}\right)$ be the mapping induced by the inverse $f^{-1}$.

Let:
 * $\forall A \in \mathcal P \left({S}\right): A = \left({f^\gets \circ f^\to}\right) \left({A}\right)$

Then $f$ is an injection.

Also see

 * Preimage of Image of Subset under Injection equals Subset
 * Subset equals Preimage of Image iff Mapping is Injection