Metamath Proof Explorer


Theorem onun2

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

Ref Expression
Assertion onun2
|- ( ( A e. On /\ B e. On ) -> ( A u. B ) e. On )

Proof

Step Hyp Ref Expression
1 ssequn1
 |-  ( A C_ B <-> ( A u. B ) = B )
2 eleq1a
 |-  ( B e. On -> ( ( A u. B ) = B -> ( A u. B ) e. On ) )
3 2 adantl
 |-  ( ( A e. On /\ B e. On ) -> ( ( A u. B ) = B -> ( A u. B ) e. On ) )
4 1 3 syl5bi
 |-  ( ( A e. On /\ B e. On ) -> ( A C_ B -> ( A u. B ) e. On ) )
5 ssequn2
 |-  ( B C_ A <-> ( A u. B ) = A )
6 eleq1a
 |-  ( A e. On -> ( ( A u. B ) = A -> ( A u. B ) e. On ) )
7 6 adantr
 |-  ( ( A e. On /\ B e. On ) -> ( ( A u. B ) = A -> ( A u. B ) e. On ) )
8 5 7 syl5bi
 |-  ( ( A e. On /\ B e. On ) -> ( B C_ A -> ( A u. B ) e. On ) )
9 eloni
 |-  ( A e. On -> Ord A )
10 eloni
 |-  ( B e. On -> Ord B )
11 ordtri2or2
 |-  ( ( Ord A /\ Ord B ) -> ( A C_ B \/ B C_ A ) )
12 9 10 11 syl2an
 |-  ( ( A e. On /\ B e. On ) -> ( A C_ B \/ B C_ A ) )
13 4 8 12 mpjaod
 |-  ( ( A e. On /\ B e. On ) -> ( A u. B ) e. On )