Cartesian Product of Unions/General Result

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Theorem

Let $I$ and $J$ be indexing sets.

Let $\family {A_i}_{i \mathop \in I}$ and $\family {B_j}_{j \mathop \in J}$ be families of sets indexed by $I$ and $J$ respectively.

Then:

$\displaystyle \paren {\bigcup_{i \mathop \in I} A_i} \times \paren {\bigcup_{j \mathop \in J} B_j} = \bigcup_{\tuple {i, j} \mathop \in I \times J} \paren {A_i \times B_j}$

where:

$\displaystyle \bigcup_{i \mathop \in I} A_i$ denotes the union of $\family {A_i}_{i \mathop \in I}$ and so on
$\times$ denotes Cartesian product.


Proof


Sources