# Finite Ordinal is equal to Natural Number

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## Theorem

Let $n$ be an element of the minimal infinite successor set.

Let $x$ be an ordinal.

Then:

- $n \sim x \implies n = x$

## Proof

Let $n \ne x$.

Then either $n < x$ or $x < n$ by Ordinal Membership is Trichotomy.

If $x < n$, then by Subset of Finite Set is Finite both $x$ and $n$ are finite.

Therefore by No Bijection between Finite Set and Proper Subset:

- $x \not \sim n$

Suppose $n < x$.

Then $x \sim n$ implies that $x$ is finite by definition.

By No Bijection between Finite Set and Proper Subset:

- $x \not \sim n$

Therefore by Rule of Transposition:

- $n \sim x \implies n = x$

$\blacksquare$

## Sources

- 1971: Gaisi Takeuti and Wilson M. Zaring:
*Introduction to Axiomatic Set Theory*: $\S 10.20$