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=                         Equisatisfiability                         =
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                             Introduction
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In logic, two formulae are equisatisfiable if the first formula is
satisfiable whenever the second is and vice versa; in other words,
either both formulae are satisfiable or both are not. Equisatisfiable
formulae may disagree, however, for a particular choice of variables.
As a result, equisatisfiability is different  from logical
equivalence, as two equivalent formulae always have the same models.

Equisatisfiability is generally used in the context of translating
formulae, so that one can define a translation to be correct if the
original and resulting formulae are equisatisfiable. Examples of
translations involving this concept are Skolemization and some
translations into conjunctive normal form.


                               Examples
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A translation from propositional logic into propositional logic in
which every binary disjunction a \vee b is replaced by ((a \vee n)
\wedge (\neg n \vee b)), where n is a new variable (one for each
replaced disjunction) is a transformation in which satisfiability is
preserved: the original and resulting formulae are equisatisfiable.
Note that these two formulae are not equivalent: the first formula has
the model in which b is true while a and n are false (the model's
truth value for n being irrelevant to the truth value of the formula),
but this is not a model of the second formula, in which n has to be
true in this case.


 License
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License URL: http://creativecommons.org/licenses/by-sa/3.0/
Original Article: http://en.wikipedia.org/wiki/Equisatisfiability


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