Definition:Image of Subset under Mapping/Definition 1

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

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

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

The image of $X$ (under $f$) is defined and denoted as:
 * $f \sqbrk X := \set {t \in T: \exists s \in X: \map f s = t}$