Isomorphic Ordinals are Equal/Proof 2
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Theorem
Let $A$ and $B$ be ordinals that are order isomorphic.
Then $A = B$.
Proof
From Well-Ordered Class is not Isomorphic to Initial Segment, neither $A$ nor $B$ can be an initial segment of the other.
By definition, every element of an ordinal is an initial segment of it.
Hence, neither $A$ nor $B$ can be an element of the other.
By Ordinal Membership is Trichotomy, it follows that $A = B$.
$\blacksquare$
Sources
- 1971: Gaisi Takeuti and Wilson M. Zaring: Introduction to Axiomatic Set Theory: $\S 7.38$