Theorem nbn2 697

Description: The negation of a wff is equivalent to the wff's equivalence to falsehood. (Contributed by Juha Arpiainen, 19-Jan-2006.) (Revised by Mario Carneiro, 31-Jan-2015.)

Assertion

Ref	Expression
nbn2	⊢ (¬ 𝜑 → (¬ 𝜓 ↔ (𝜑 ↔ 𝜓)))

Proof of Theorem nbn2

Step	Hyp	Ref	Expression
1		pm5.21im 696	. 2 ⊢ (¬ 𝜑 → (¬ 𝜓 → (𝜑 ↔ 𝜓)))
2		biimpr 130	. . 3 ⊢ ((𝜑 ↔ 𝜓) → (𝜓 → 𝜑))
3		mtt 685	. . 3 ⊢ (¬ 𝜑 → (¬ 𝜓 ↔ (𝜓 → 𝜑)))
4	2, 3	syl5ibr 156	. 2 ⊢ (¬ 𝜑 → ((𝜑 ↔ 𝜓) → ¬ 𝜓))
5	1, 4	impbid 129	1 ⊢ (¬ 𝜑 → (¬ 𝜓 ↔ (𝜑 ↔ 𝜓)))

Syntax hints:  ¬ wn 3   → wi 4   ↔ wb 105

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 106  ax-ia2 107  ax-ia3 108  ax-in1 614  ax-in2 615

This theorem depends on definitions:  df-bi 117

This theorem is referenced by:  bibif  698  pm5.18dc  883  biassdc  1395