Definition:Class Union/General Definition

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Let $A$ be a class.

The union of $A$ is:

$\bigcup A := \set {x: \exists y: x \in y \land y \in A}$

That is, the class of all elements of all elements of $A$ which are themselves sets.

Also denoted as

Some sources denote $\bigcup A$ as $\ds \bigcup_{x \mathop \in A} x$.

Also see

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