Theorem pm4.72 949

Description: Implication in terms of biconditional and disjunction. Theorem *4.72 of [WhiteheadRussell] p. 121. (Contributed by NM, 30-Aug-1993.) (Proof shortened by Wolf Lammen, 30-Jan-2013.)

Assertion

Ref	Expression
pm4.72	⊢ ((𝜑 → 𝜓) ↔ (𝜓 ↔ (𝜑 ∨ 𝜓)))

Proof of Theorem pm4.72

Step	Hyp	Ref	Expression
1		olc 867	. . 3 ⊢ (𝜓 → (𝜑 ∨ 𝜓))
2		pm2.621 898	. . 3 ⊢ ((𝜑 → 𝜓) → ((𝜑 ∨ 𝜓) → 𝜓))
3	1, 2	impbid2 229	. 2 ⊢ ((𝜑 → 𝜓) → (𝜓 ↔ (𝜑 ∨ 𝜓)))
4		orc 866	. . 3 ⊢ (𝜑 → (𝜑 ∨ 𝜓))
5		biimpr 223	. . 3 ⊢ ((𝜓 ↔ (𝜑 ∨ 𝜓)) → ((𝜑 ∨ 𝜓) → 𝜓))
6	4, 5	syl5 34	. 2 ⊢ ((𝜓 ↔ (𝜑 ∨ 𝜓)) → (𝜑 → 𝜓))
7	3, 6	impbii 212	1 ⊢ ((𝜑 → 𝜓) ↔ (𝜓 ↔ (𝜑 ∨ 𝜓)))

Syntax hints:   → wi 4   ↔ wb 209   ∨ wo 846

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 210  df-or 847

This theorem is referenced by:  bigolden  1026  cadan  1615  ssequn1  4070  ssunsn2  4715  vtxd0nedgb  27430  bj-consensusALT  34398  wl-ifpimpr  35260  elpaddn0  37437