Theorem 1on

Description: Ordinal 1 is an ordinal number. (Contributed by NM, 29-Oct-1995)

	Ref	Expression
Assertion	1on	⊢ 1o ∈ On

Proof

Step	Hyp	Ref	Expression
1		df-1o	⊢ 1o = suc ∅
2		0elon	⊢ ∅ ∈ On
3	2	onsuci	⊢ suc ∅ ∈ On
4	1 3	eqeltri	⊢ 1o ∈ On