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logical equivalence

Two sentences that have the same truth value on every row.

Two sentences A and B are logically equivalent, written A ≡ B, when they have the same truth value on every valuation. Equivalently, the biconditional A ↔ B is a tautology.

Examples

¬(P ∧ Q) ≡ ¬P ∨ ¬Q.