Direct Image Mapping of Domain is Image Set of Mapping
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Theorem
Let $S$ and $T$ be sets.
Let $\powerset S$ and $\powerset T$ be their power sets.
Let $f \subseteq S \times T$ be a mapping from $S$ to $T$.
Let $f^\to: \powerset S \to \powerset T$ be the direct image mapping of $f$:
- $\forall X \in \powerset S: \map {f^\to} X = \begin {cases} \set {t \in T: \exists s \in X: \map f s = t} & : X \ne \O \\ \O & : X = \O \end {cases}$
Then:
- $\map {f^\to} S = \Img f$
where $\Img f$ is the image set of $f$.
Proof
\(\ds y\) | \(\in\) | \(\ds \map {f^\to} S\) | ||||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds \exists x \in S: \, \) | \(\ds \map f x\) | \(=\) | \(\ds y\) | Definition of Direct Image Mapping of Mapping | |||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds y\) | \(\in\) | \(\ds \Img f\) | Definition of Image Set of Mapping |
$\blacksquare$