Theorem 1oex

Description: Ordinal 1 is a set. (Contributed by BJ, 6-Apr-2019) (Proof shortened by AV, 1-Jul-2022) Remove dependency on ax-10 , ax-11 , ax-12 , ax-un . (Revised by Zhi Wang, 19-Sep-2024)

	Ref	Expression
Assertion	1oex	⊢ 1o ∈ V

Proof

Step	Hyp	Ref	Expression
1		df1o2	⊢ 1o = { ∅ }
2		snex	⊢ { ∅ } ∈ V
3	1 2	eqeltri	⊢ 1o ∈ V