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sav08:qe_from_conjunction_of_literals_suffices [2009/04/21 19:04] vkuncak |
sav08:qe_from_conjunction_of_literals_suffices [2015/04/21 17:30] |
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| - | ====== Quantifier Elimination from Conjunction of Literals is Sufficient ====== | ||
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| - | To show constructively that a theory has quantifier elimination, it suffices to show that we can eliminate an existential quantifier applied to a conjunction of [[wp>literal (mathematical logic)|literal]]s, that is, show that each formula of the form: | ||
| - | \[ | ||
| - | \exists x. \bigwedge_{i=1}^n L_i | ||
| - | \] | ||
| - | where each $L_i$ is a literal, is equivalent to a quantifier-free formula. Indeed, suppose we know how to eliminate quantifiers from conjunctions of formulae, then if $F$ is a quantifier-free formula, we can write it in [[wp>disjunctive normal form]] | ||
| - | \[ | ||
| - | \bigvee_{j=1}^m \bigwedge_{i=1}^n L_{ij} | ||
| - | \] | ||
| - | and use the fact that | ||
| - | \[ | ||
| - | \exists x. \bigvee_{j=1}^m \bigwedge_{i=1}^n L_{ij} | ||
| - | \] | ||
| - | is equivalent to | ||
| - | \[ | ||
| - | \bigvee_{j=1}^m \exists x. \bigwedge_{i=1}^n L_{ij} | ||
| - | \] | ||
| - | |||
| - | Finally, to eliminate a universal quantifier | ||
| - | \[ | ||
| - | \forall x. F | ||
| - | \] | ||
| - | where $F$ is quantifier-free, we transform $\lnot F$ into disjunctive normal form, and use the fact that $\forall x. F$ is equivalent to $\lnot \exists x. \lnot F.$ | ||
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| - | ((taken from [[wp>Quantifier Elimination]] to which it was originally contributed, so the Wikipedia sharing rights apply)) | ||