Theorem nbn 688

Description: The negation of a wff is equivalent to the wff's equivalence to falsehood. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Wolf Lammen, 3-Oct-2013.)

Hypothesis

Ref	Expression
nbn.1	⊢ ¬ 𝜑

Assertion

Ref	Expression
nbn	⊢ (¬ 𝜓 ↔ (𝜓 ↔ 𝜑))

Proof of Theorem nbn

Step	Hyp	Ref	Expression
1		nbn.1	. . 3 ⊢ ¬ 𝜑
2		bibif 687	. . 3 ⊢ (¬ 𝜑 → ((𝜓 ↔ 𝜑) ↔ ¬ 𝜓))
3	1, 2	ax-mp 5	. 2 ⊢ ((𝜓 ↔ 𝜑) ↔ ¬ 𝜓)
4	3	bicomi 131	1 ⊢ (¬ 𝜓 ↔ (𝜓 ↔ 𝜑))

Syntax hints:  ¬ wn 3   ↔ wb 104

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 105  ax-ia2 106  ax-ia3 107  ax-in1 603  ax-in2 604

This theorem depends on definitions:  df-bi 116

This theorem is referenced by:  nbn3  689  nbfal  1342  n0rf  3375  eq0  3381  disj  3411  dm0rn0  4756  reldm0  4757