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Logical equivalence

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In logic, statements p and q are logically equivalent if they have the same logical content.

Syntactically[?], p and q are equivalent if each can be proved from the other. Semantically[?], p and q are equivalent if they have the same truth value in every model.

Logical equivalence is often confused with material equivalence. The former is a statement in the metalanguage[?], claiming something about statements p and q in the object language[?]. But the material equivalence of p and q (often written "pq") is itself another statement in the object language. There is a relationship, however; p and q are syntactically equivalent if and only if pq is a theorem, while p and q are semantically equivalent if and only if pq is a tautology.

Logical equivalence is sometimes denoted pq or pq. However, the latter notation is also used for material equivalence.



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