Image of Union under Mapping/Family of Sets

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

Let $\family {S_i}_{i \mathop \in I}$ be a family of subsets of $S$.

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

Then:
 * $\displaystyle f \sqbrk {\bigcup_{i \mathop \in I} S_i} = \bigcup_{i \mathop \in I} f \sqbrk {S_i}$

where $\displaystyle \bigcup_{i \mathop \in I} S_i$ denotes the union of $\family {S_i}_{i \mathop \in I}$.

Proof
As $f$, being a mapping, is also a relation, we can apply Image of Union under Relation: Family of Sets:


 * $\displaystyle \mathcal R \sqbrk {\bigcup_{i \mathop \in I} S_i} = \bigcup_{i \mathop \in I} \mathcal R \sqbrk {S_i}$