Definition:Class Union

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Let $A$ and $B$ be two classes.

The (class) union $A \cup B$ of $A$ and $B$ is defined as the class of all sets $x$ such that either $x \in A$ or $x \in B$ or both:

$x \in A \cup B \iff x \in A \lor x \in B$


$A \cup B = \set {x: x \in A \lor x \in B}$

General Definition

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 see

  • Results about class unions can be found here.


Union is translated:

In French: somme  (literally: sum)
In French: union
In French: réunion
In Dutch: vereniging