Union of Inverse of Relations is Inverse of their Union
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Theorem
For $i \in \set {1, 2}$, let $\RR_i \subseteq S_i \times T_i$ be relations on $S_i \times T_i$.
Let ${\RR_i}^{-1} \subseteq T_i \times S_i$ be the inverse of $\RR_i$.
Then:
- ${\RR_1}^{-1} \cup {\RR_2}^{-1} = \paren {\RR_1 \cup \RR_2}^{-1}$
Proof
Let $\tuple {t, s} \in {\RR_1}^{-1} \cup {\RR_2}^{-1}$.
By definition of union:
- $\tuple {t, s} \in {\RR_1}^{-1} \lor \tuple {t, s} \in {\RR_2}^{-1}$.
For $i \in \set {1, 2}$, let $\tuple {t, s} \in {\RR_i}^{-1}$.
By definition of inverse:
- $\tuple {s, t} \in \RR_i$
That is:
- $\tuple {s, t} \in \RR_1 \lor \tuple {s, t} \in \RR_2$
By definition of union:
- $\tuple {s, t} \in \RR_1 \lor \tuple {s, t} \in \RR_2 \iff \tuple {s, t} \in \RR_1 \cup \RR_2$
By the definition of inverse:
- $\tuple {t, s} \in \paren {\RR_1 \cup \RR_2}^{-1}$
$\blacksquare$