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Metamath Proof Explorer

Theorem pm4.53

Description: Theorem *4.53 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

Ref Expression
Assertion pm4.53 ⊢ ¬ φ ∧ ¬ ψ ↔ ¬ φ ∨ ψ

Proof

Step Hyp Ref Expression
1 pm4.52 ⊢ φ ∧ ¬ ψ ↔ ¬ ¬ φ ∨ ψ
2 1 con2bii ⊢ ¬ φ ∨ ψ ↔ ¬ φ ∧ ¬ ψ
3 2 bicomi ⊢ ¬ φ ∧ ¬ ψ ↔ ¬ φ ∨ ψ