Image of Subset is Image of Restriction

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

Let $X \subseteq S$.

Let $f {\restriction_X}$ be the restriction of $f$ to $X$.

Then:
 * $f \sqbrk X = \Img {f {\restriction_X} }$

where $\Img f$ denotes the image of $f$, defined as:
 * $\Img f = \set {t \in T: \exists s \in S: t = \map f s}$

Proof
Let $y \in f \sqbrk X$.

Then by definition of the image of a subset:
 * $\exists x \in X: \map f x = y$

or equivalently:
 * $\exists x \in X: \tuple {x, y} \in f$

But then by definition of restriction of $f$ to $X$:
 * $f {\restriction_X} = \set {\tuple {x, y} \in f: x \in X}$

Thus by definition of the image set of $f {\restriction_X}$:


 * $\Img {f {\restriction_X} } = f {\restriction_X} \sqbrk X = \set {y \in T: \exists x \in X: \tuple {x, y} \in f {\restriction_X} }$

Hence it can be seen that:
 * $y \in \Img {f {\restriction_X} }$

So:
 * $f \sqbrk X \subseteq \Img {f {\restriction_X} }$

Now suppose that $y \in \Img {f {\restriction_X} }$.

Then by definition of the image set of $f {\restriction_X}$:
 * $\exists x \in S: \tuple {x, y} \in f {\restriction_X}$

By definition of restriction of $f$ to $X$:
 * $x \in X$

Thus by definition of image of a subset:
 * $y \in f \sqbrk X$

So:
 * $\Img {f {\restriction_X} } \subseteq f \sqbrk X$

We have:
 * $f \sqbrk X \subseteq \Img {f {\restriction_X} }$
 * $\Img {f {\restriction_X} } \subseteq f \sqbrk X$

The result follows by definition of set equality.