Restriction of Injection is Injection

Theorem
Let $f: S \to T$ be an injection.

Let $X \subseteq S$ be a subset of $S$.

Let $f \sqbrk X$ denote the image of $X$ under $f$.

Let $Y \subseteq T$ be a subset of $T$ such that $f \sqbrk X \subseteq Y$.

The restriction $f \restriction_{X \times Y}$ of $f$ to $X \times Y$ is an injection from $X$ to $Y$.

Proof
First we show that $f \restriction_{X \times Y}$ is a mapping from $X$ to $Y$.

By Restriction of Mapping is Mapping, $f \restriction_{X \times T}$ is a mapping from $X$ to $T$.

If $x \in X$, then by the definition of image:
 * $\map f x \in f \sqbrk X$

Since $f \sqbrk X \subseteq Y$, $f \restriction_{X \times Y}$ is a mapping from $X$ to $Y$.

By definition of an injection:


 * $\forall s_1, s_2 \in S: \map f {s_1} = \map f {s_2} \implies s_1 = s_2$.

$f \restriction_{X \times Y}: X \to Y$ were not an injection.

Then:
 * $\exists x_1, x_2 \in X: x_1 \ne x_2, \map f {x_1} = \map f {x_2}$

But then:
 * $\exists x_1, x_2 \in S: x_1 \ne x_2, \map f {x_1} = \map f {x_2}$

So $f: S \to T$ would not then be an injection.

Hence the result.