Theorem notbi 319

Description: Contraposition. Theorem *4.11 of [WhiteheadRussell] p. 117. (Contributed by NM, 21-May-1994.) (Proof shortened by Wolf Lammen, 12-Jun-2013.)

Assertion

Ref	Expression
notbi	⊢ ((𝜑 ↔ 𝜓) ↔ (¬ 𝜑 ↔ ¬ 𝜓))

Proof of Theorem notbi

Step	Hyp	Ref	Expression
1		id 22	. . 3 ⊢ ((𝜑 ↔ 𝜓) → (𝜑 ↔ 𝜓))
2	1	notbid 318	. 2 ⊢ ((𝜑 ↔ 𝜓) → (¬ 𝜑 ↔ ¬ 𝜓))
3		id 22	. . 3 ⊢ ((¬ 𝜑 ↔ ¬ 𝜓) → (¬ 𝜑 ↔ ¬ 𝜓))
4	3	con4bid 317	. 2 ⊢ ((¬ 𝜑 ↔ ¬ 𝜓) → (𝜑 ↔ 𝜓))
5	2, 4	impbii 209	1 ⊢ ((𝜑 ↔ 𝜓) ↔ (¬ 𝜑 ↔ ¬ 𝜓))

Syntax hints:  ¬ wn 3   ↔ wb 206

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8

This theorem depends on definitions:  df-bi 207

This theorem is referenced by:  notbii  320  con4bii  321  con2bi  353  nbn2  370  pm5.32  573  norass  1534  hadnot  1599  had0  1601  cbvexdw  2345  cbvexd  2416  rexbiOLD  3111  rexprg  4721  isocnv3  7368  suppimacnv  8215  sumodd  16436  f1omvdco3  19491  ist0cld  33779  onsuct0  36407  bj-cbvexdv  36766  wl-3xornot  37447  ifpbi1  43439  ifpbi13  43451  abciffcbatnabciffncba  46844  abciffcbatnabciffncbai  46845  ichn  47330