Theorem oneluni 6433

Description: An ordinal number equals its union with any element. (Contributed by NM, 13-Jun-1994.)

Hypothesis

Ref	Expression
on.1	⊢ 𝐴 ∈ On

Assertion

Ref	Expression
oneluni	⊢ (𝐵 ∈ 𝐴 → (𝐴 ∪ 𝐵) = 𝐴)

Proof of Theorem oneluni

Step	Hyp	Ref	Expression
1		on.1	. . 3 ⊢ 𝐴 ∈ On
2	1	onelssi 6429	. 2 ⊢ (𝐵 ∈ 𝐴 → 𝐵 ⊆ 𝐴)
3		ssequn2 4121	. 2 ⊢ (𝐵 ⊆ 𝐴 ↔ (𝐴 ∪ 𝐵) = 𝐴)
4	2, 3	sylib 219	1 ⊢ (𝐵 ∈ 𝐴 → (𝐴 ∪ 𝐵) = 𝐴)

Syntax hints:   → wi 4   = wceq 1543   ∈ wcel 2115   ∪ cun 3884   ⊆ wss 3886  Oncon0 6313

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1798  ax-4 1812  ax-5 1913  ax-6 1970  ax-7 2011  ax-8 2117  ax-9 2125  ax-ext 2708

This theorem depends on definitions:  df-bi 208  df-an 397  df-or 850  df-tru 1546  df-ex 1783  df-sb 2070  df-clab 2715  df-cleq 2728  df-clel 2811  df-ral 3051  df-v 3430  df-un 3891  df-ss 3903  df-uni 4842  df-tr 5183  df-po 5529  df-so 5530  df-fr 5574  df-we 5576  df-ord 6316  df-on 6317

This theorem is referenced by:  omabs2  43774