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2. Logical equivalence - Wikipedia

   en.wikipedia.org/wiki/Logical_equivalence

   In logic and mathematics, statements and are said to be logically equivalent if they have the same truth value in every model. [1] The logical equivalence of p {\displaystyle p} and q {\displaystyle q} is sometimes expressed as p ≡ q {\displaystyle p\equiv q} , p :: q {\displaystyle p::q} , E p q {\displaystyle {\textsf {E}}pq} , or p q ...

3. T-schema - Wikipedia

   en.wikipedia.org/wiki/T-schema

   For example, the sentence "'Snow is white' is true" becomes materially equivalent with the sentence "snow is white", i.e. 'snow is white' is true if and only if snow is white. Said again, a sentence of the form "A" is true if and only if A is true. The truth of more complex sentences is defined in terms of the components of the sentence:

4. If and only if - Wikipedia

   en.wikipedia.org/wiki/If_and_only_if

   The corresponding logical symbols are "", "", [6] and , [10] and sometimes "iff".These are usually treated as equivalent. However, some texts of mathematical logic (particularly those on first-order logic, rather than propositional logic) make a distinction between these, in which the first, ↔, is used as a symbol in logic formulas, while ⇔ is used in reasoning about those logic formulas ...

5. Sentence (mathematical logic) - Wikipedia

   en.wikipedia.org/wiki/Sentence_(mathematical_logic)

   A sentence can be viewed as expressing a proposition, something that must be true or false. The restriction of having no free variables is needed to make sure that sentences can have concrete, fixed truth values : as the free variables of a (general) formula can range over several values, the truth value of such a formula may vary.

6. Logical truth - Wikipedia

   en.wikipedia.org/wiki/Logical_truth

   A statement is logically true if, and only if its opposite is logically false. The opposite statements must contradict one another. In this way all logical connectives can be expressed in terms of preserving logical truth. The logical form of a sentence is determined by its semantic or syntactic structure and by the placement of logical constants.

7. Glossary of logic - Wikipedia

   en.wikipedia.org/wiki/Glossary_of_logic

   logically equivalent Referring to statements that have the same truth value in every possible scenario, indicating that they are interchangeable in logical reasoning. logicism The philosophical belief that mathematics can be reduced to logic and that all mathematical truths can be derived from logical axioms and definitions. Löwenheim–Skolem ...