Theorem dfbi3 933

Description: An alternate definition of the biconditional. Theorem *5.23 of [WhiteheadRussell] p. 124. (Contributed by NM, 27-Jun-2002.) (Proof shortened by Wolf Lammen, 3-Nov-2013.)

Assertion

Ref	Expression
dfbi3	⊢ ((𝜑 ↔ 𝜓) ↔ ((𝜑 ∧ 𝜓) ∨ (¬ 𝜑 ∧ ¬ 𝜓)))

Proof of Theorem dfbi3

Step	Hyp	Ref	Expression
1		xor 931	. 2 ⊢ (¬ (𝜑 ↔ ¬ 𝜓) ↔ ((𝜑 ∧ ¬ ¬ 𝜓) ∨ (¬ 𝜓 ∧ ¬ 𝜑)))
2		pm5.18 370	. 2 ⊢ ((𝜑 ↔ 𝜓) ↔ ¬ (𝜑 ↔ ¬ 𝜓))
3		notnotb 303	. . . 4 ⊢ (𝜓 ↔ ¬ ¬ 𝜓)
4	3	anbi2i 726	. . 3 ⊢ ((𝜑 ∧ 𝜓) ↔ (𝜑 ∧ ¬ ¬ 𝜓))
5		ancom 465	. . 3 ⊢ ((¬ 𝜑 ∧ ¬ 𝜓) ↔ (¬ 𝜓 ∧ ¬ 𝜑))
6	4, 5	orbi12i 542	. 2 ⊢ (((𝜑 ∧ 𝜓) ∨ (¬ 𝜑 ∧ ¬ 𝜓)) ↔ ((𝜑 ∧ ¬ ¬ 𝜓) ∨ (¬ 𝜓 ∧ ¬ 𝜑)))
7	1, 2, 6	3bitr4i 291	1 ⊢ ((𝜑 ↔ 𝜓) ↔ ((𝜑 ∧ 𝜓) ∨ (¬ 𝜑 ∧ ¬ 𝜓)))

Syntax hints:  ¬ wn 3   ↔ wb 195   ∨ wo 382   ∧ wa 383

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 196  df-or 384  df-an 385

This theorem is referenced by:  pm5.24  934  4exmid  977  nanbi  1446  ifbi  4057  sqf11  24665  bj-dfbi4  31728  raaan2  39824  2reu4a  39838