Metamath Proof Explorer


Theorem unon

Description: The class of all ordinal numbers is its own union. Exercise 11 of TakeutiZaring p. 40. (Contributed by NM, 12-Nov-2003)

Ref Expression
Assertion unon On = On

Proof

Step Hyp Ref Expression
1 eluni2 ( 𝑥 On ↔ ∃ 𝑦 ∈ On 𝑥𝑦 )
2 onelon ( ( 𝑦 ∈ On ∧ 𝑥𝑦 ) → 𝑥 ∈ On )
3 2 rexlimiva ( ∃ 𝑦 ∈ On 𝑥𝑦𝑥 ∈ On )
4 1 3 sylbi ( 𝑥 On → 𝑥 ∈ On )
5 vex 𝑥 ∈ V
6 5 sucid 𝑥 ∈ suc 𝑥
7 suceloni ( 𝑥 ∈ On → suc 𝑥 ∈ On )
8 elunii ( ( 𝑥 ∈ suc 𝑥 ∧ suc 𝑥 ∈ On ) → 𝑥 On )
9 6 7 8 sylancr ( 𝑥 ∈ On → 𝑥 On )
10 4 9 impbii ( 𝑥 On ↔ 𝑥 ∈ On )
11 10 eqriv On = On