Preimage of Union under Mapping/Family of Sets/Proof 2

Proof
We have that $f$ is a mapping, and so also a relation.

Thus its inverse $f^{-1}$ is also a relation.

Hence we can apply Image of Union under Relation: Family of Sets:


 * $\displaystyle \RR \sqbrk {\bigcup_{i \mathop \in I} T_i} = \bigcup_{i \mathop \in I} \RR \sqbrk {T_i}$

where $\RR \sqbrk {T_i}$ denotes the image of $T_i$ under $\RR$.