Description: Define the biconditional as conditional logic operator. (Contributed by RP, 20-Apr-2020) (Proof shortened by Wolf Lammen, 30-Apr-2024)
Ref | Expression | ||
---|---|---|---|
Assertion | ifpdfbi | ⊢ ( ( 𝜑 ↔ 𝜓 ) ↔ if- ( 𝜑 , 𝜓 , ¬ 𝜓 ) ) |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | con34b | ⊢ ( ( 𝜓 → 𝜑 ) ↔ ( ¬ 𝜑 → ¬ 𝜓 ) ) | |
2 | 1 | anbi2i | ⊢ ( ( ( 𝜑 → 𝜓 ) ∧ ( 𝜓 → 𝜑 ) ) ↔ ( ( 𝜑 → 𝜓 ) ∧ ( ¬ 𝜑 → ¬ 𝜓 ) ) ) |
3 | dfbi2 | ⊢ ( ( 𝜑 ↔ 𝜓 ) ↔ ( ( 𝜑 → 𝜓 ) ∧ ( 𝜓 → 𝜑 ) ) ) | |
4 | dfifp2 | ⊢ ( if- ( 𝜑 , 𝜓 , ¬ 𝜓 ) ↔ ( ( 𝜑 → 𝜓 ) ∧ ( ¬ 𝜑 → ¬ 𝜓 ) ) ) | |
5 | 2 3 4 | 3bitr4i | ⊢ ( ( 𝜑 ↔ 𝜓 ) ↔ if- ( 𝜑 , 𝜓 , ¬ 𝜓 ) ) |