Theorem onun2

Description: The union of two ordinals is an ordinal. (Contributed by Scott Fenton, 9-Aug-2024)

	Ref	Expression
Assertion	onun2	⊢ ( ( 𝐴 ∈ On ∧ 𝐵 ∈ On ) → ( 𝐴 ∪ 𝐵 ) ∈ On )

Proof

Step	Hyp	Ref	Expression
1		ssequn1	⊢ ( 𝐴 ⊆ 𝐵 ↔ ( 𝐴 ∪ 𝐵 ) = 𝐵 )
2		eleq1a	⊢ ( 𝐵 ∈ On → ( ( 𝐴 ∪ 𝐵 ) = 𝐵 → ( 𝐴 ∪ 𝐵 ) ∈ On ) )
3	2	adantl	⊢ ( ( 𝐴 ∈ On ∧ 𝐵 ∈ On ) → ( ( 𝐴 ∪ 𝐵 ) = 𝐵 → ( 𝐴 ∪ 𝐵 ) ∈ On ) )
4	1 3	syl5bi	⊢ ( ( 𝐴 ∈ On ∧ 𝐵 ∈ On ) → ( 𝐴 ⊆ 𝐵 → ( 𝐴 ∪ 𝐵 ) ∈ On ) )
5		ssequn2	⊢ ( 𝐵 ⊆ 𝐴 ↔ ( 𝐴 ∪ 𝐵 ) = 𝐴 )
6		eleq1a	⊢ ( 𝐴 ∈ On → ( ( 𝐴 ∪ 𝐵 ) = 𝐴 → ( 𝐴 ∪ 𝐵 ) ∈ On ) )
7	6	adantr	⊢ ( ( 𝐴 ∈ On ∧ 𝐵 ∈ On ) → ( ( 𝐴 ∪ 𝐵 ) = 𝐴 → ( 𝐴 ∪ 𝐵 ) ∈ On ) )
8	5 7	syl5bi	⊢ ( ( 𝐴 ∈ On ∧ 𝐵 ∈ On ) → ( 𝐵 ⊆ 𝐴 → ( 𝐴 ∪ 𝐵 ) ∈ On ) )
9		eloni	⊢ ( 𝐴 ∈ On → Ord 𝐴 )
10		eloni	⊢ ( 𝐵 ∈ On → Ord 𝐵 )
11		ordtri2or2	⊢ ( ( Ord 𝐴 ∧ Ord 𝐵 ) → ( 𝐴 ⊆ 𝐵 ∨ 𝐵 ⊆ 𝐴 ) )
12	9 10 11	syl2an	⊢ ( ( 𝐴 ∈ On ∧ 𝐵 ∈ On ) → ( 𝐴 ⊆ 𝐵 ∨ 𝐵 ⊆ 𝐴 ) )
13	4 8 12	mpjaod	⊢ ( ( 𝐴 ∈ On ∧ 𝐵 ∈ On ) → ( 𝐴 ∪ 𝐵 ) ∈ On )