Let $M$ be the smallest set of ordinals satisfying
- $0\in M$
- $x\in M\implies x+1\in M$
- $S\subseteq M\implies \bigcup S\in M$
What does $M$ look like? Is it all ordinals? Is it all countable ordinals?
Is there a standard notation for $M$?
2026-04-02 13:32:24.1775136744
Ordinals defined by successor and union
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