Metamath Proof Explorer


Theorem ondif1

Description: Two ways to say that A is a nonzero ordinal number. (Contributed by Mario Carneiro, 21-May-2015)

Ref Expression
Assertion ondif1 ( 𝐴 ∈ ( On ∖ 1o ) ↔ ( 𝐴 ∈ On ∧ ∅ ∈ 𝐴 ) )

Proof

Step Hyp Ref Expression
1 dif1o ( 𝐴 ∈ ( On ∖ 1o ) ↔ ( 𝐴 ∈ On ∧ 𝐴 ≠ ∅ ) )
2 on0eln0 ( 𝐴 ∈ On → ( ∅ ∈ 𝐴𝐴 ≠ ∅ ) )
3 2 pm5.32i ( ( 𝐴 ∈ On ∧ ∅ ∈ 𝐴 ) ↔ ( 𝐴 ∈ On ∧ 𝐴 ≠ ∅ ) )
4 1 3 bitr4i ( 𝐴 ∈ ( On ∖ 1o ) ↔ ( 𝐴 ∈ On ∧ ∅ ∈ 𝐴 ) )