Image of Intersection under Injection/General Result

Theorem
Let $S$ and $T$ be sets.

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

Let $\mathcal P \left({S}\right)$ be the power set of $S$.

Then:
 * $\displaystyle \forall \mathbb S \subseteq \mathcal P \left({S}\right): f \left({\bigcap \mathbb S}\right) = \bigcap_{X \mathop \in \mathbb S} f \left({X}\right)$

iff $f$ is an injection.

Proof
An injection is a type of one-to-one relation, and therefore also a one-to-many relation.

Therefore One-to-Many Image of Intersections applies:


 * $\displaystyle \forall \mathbb S \subseteq \mathcal P \left({S}\right): \mathcal R \left({\bigcap \mathbb S}\right) = \bigcap_{X \mathop \in \mathbb S} \mathcal R \left({\mathbb S}\right)$

iff $\mathcal R$ is a one-to-many relation.

We have that $f$ is a mapping and therefore a many-to-one relation.

So $f$ is a one-to-many relation iff $f$ is also an injection.

It follows that:
 * $\displaystyle \forall \mathbb S \subseteq \mathcal P \left({S}\right): f \left({\bigcap \mathbb S}\right) = \bigcap_{X \mathop \in \mathbb S} f \left({X}\right)$

iff $f$ is an injection.