Definition:Cartesian Product/Cartesian Space/Family of Sets/Definition 2

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Let $I$ be an indexing set.

Let $\family {S_i}_{i \mathop \in I}$ be an family of sets indexed by $I$.

Let $\ds \prod_{i \mathop \in I} S_i$ be the Cartesian product of $\family {S_i}_{i \mathop \in I}$.

Let $S$ be a set such that:

$\forall i \in I: S_i = S$

The Cartesian space of $S$ indexed by $I$ is defined and denoted as:

$\ds S^I := \set {f: \paren {f: I \to S} \land \paren {\forall i \in I: \paren {\map f i \in S} } }$

Also see

This follows from Set Union is Idempotent:

$\ds \bigcup_{i \mathop \in I} S = S$