Theorem 2onn

Description: The ordinal 2 is a natural number. (Contributed by NM, 28-Sep-2004)

	Ref	Expression
Assertion	2onn	⊢ 2o ∈ ω

Proof

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