Theorem 5onn

Description: The ordinal 5 is a natural number. (Contributed by BTernaryTau, 4-Sep-2026)

	Ref	Expression
Assertion	5onn	⊢ 5o ∈ ω

Proof

Step	Hyp	Ref	Expression
1		df-5o	⊢ 5o = suc 4o
2		4onn	⊢ 4o ∈ ω
3		peano2	⊢ ( 4o ∈ ω → suc 4o ∈ ω )
4	2 3	ax-mp	⊢ suc 4o ∈ ω
5	1 4	eqeltri	⊢ 5o ∈ ω