Union of Transitive Class is Subclass
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Theorem
Let $A$ be a transitive class.
Let $\bigcup A$ denote the union of $A$.
Then:
- $\bigcup A \subseteq A$
Proof
Let $A$ be transitive.
Let $x \in \bigcup A$.
Then by definition:
- $\exists y \in A: x \in y$
By definition of transitive class:
- $x \in y \land y \in A \implies x \in A$
and so:
- $x \in A$
Hence the result by definition of subclass.
$\blacksquare$
Sources
- 2010: Raymond M. Smullyan and Melvin Fitting: Set Theory and the Continuum Problem (revised ed.) ... (previous) ... (next): Chapter $2$: Some Basics of Class-Set Theory: $\S 5$ The union axiom: Exercise $5.3. \ \text {(a)}$