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Dating maintenance tags: {{Dn}}

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	I'~~m Yoshiko Oquendo~~. ~~He presently life~~ in ~~Arizona and~~ his ~~mothers~~ and ~~fathers live nearby~~. ~~Interviewing~~ is ~~what she does~~. ~~What he truly enjoys doing~~ is to ~~perform handball but he~~ is ~~having difficulties to discover time for it~~.<br><br>~~Look into my webpage [http~~://~~Www~~.~~Tobynabors~~.~~com~~/~~ActivityFeed~~/~~MyProfile~~/~~tabid~~/61/~~userId~~/~~46195~~/~~Default~~.~~aspx Www~~.~~Tobynabors~~.~~com~~]		The '''Herbrandization''' of a logical formula (named after [[Jacques Herbrand]]) is a construction that is dual to the [[Skolemization]] of a formula. [[Thoralf Skolem]] had considered the Skolemizations of formulas in prenex form as part of his proof of the [[Löwenheim-Skolem theorem]] (Skolem 1920). Herbrand worked with this dual notion of Herbrandization, generalized to apply to non-prenex formulas as well, in order to prove [[Herbrand's theorem (proof theory)\|Herbrand's theorem]] (Herbrand 1930).

			The resulting formula is not necessarily [[logical equivalence\|equivalent]] to the original one. As with Skolemization which only preserves [[satisfiability]], Herbrandization being Skolemization's dual preserves [[validity]]: the resulting formula is valid if and only if the original one is.

			Let <math>F</math> be a formula in the language of [[first-order logic]]. We may assume that <math>F</math> contains no variable that is bound by two different quantifier occurrences, and that no variable occurs both bound and free. (That is, <math>F</math> could be relettered to ensure these conditions, in such a way that the result is an equivalent formula).

			The ''Herbrandization'' of <math>F</math> is then obtained as follows:

			* First, replace any free variables in <math>F</math> by constant symbols.

			* Second, delete all quantifiers on variables that are either (1) universally quantified and within an even number of negation signs, or (2) existentially quantified and within an odd number of negation signs.

			* Finally, replace each such variable <math>v</math> with a function symbol <math>f_v(x_1,\dots,x_k)</math>, where <math>x_1,\dots,x_k</math> are the variables that are still quantified, and whose quantifiers govern <math>v</math>.

			For instance, consider the formula <math>F := \forall y \exists x [R(y,x) \wedge \neg\exists z S(x,z)]</math>. There are no free variables to replace. The variables <math>y,z</math> are the kind we consider for the second step, so we delete the quantifiers <math>\forall y</math> and <math>\exists z</math>. Finally, we then replace <math>y</math> with a constant <math>c_y</math> (since there were no other quantifiers governing <math>y</math>), and we replace <math>z</math> with a function symbol <math>f_z(x)</math>:

			: <math> F^H = \exists x [R(c_y,x) \wedge \neg S(x,f_z(x))]. </math>

			The ''Skolemization'' of a formula is obtained similarly, except that in the second step above, we would delete quantifiers on variables that are either (1) existentially quantified and within an even number of negations, or (2) universally quantified and within an odd number of negations. Thus, considering the same <math>F</math> from above, its Skolemization would be:

			: <math> F^S = \forall y [R(y,f_x(y)) \wedge \neg\exists z S(f_x(y),z)]. </math>

			To understand the significance of these constructions, see [[Herbrand's theorem (proof theory)\|Herbrand's theorem]] or the [[Löwenheim-Skolem theorem]].

			== See also ==
			* [[Predicate functor logic]]

			==References==

			* Skolem, T. "Logico-combinatorial investigations in the satisfiability or provability of mathematical propositions: A simplified proof of a theorem by L. Löwenheim and generalizations of the theorem". (In van Heijenoort 1967, 252-63.)

			* Herbrand, J. "Investigations in proof theory: The properties of true propositions". (In van Heijenoort 1967, 525-81.)

			* van Heijenoort, J. ''From Frege to Gödel: A Source Book in Mathematical Logic, 1879-1931''. Harvard University Press, 1967.

			[[Category:Logic]]
			[[Category:Normal forms (logic)]]

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remove Erik9bot category,outdated, tag and general fixes

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I'm Yoshiko Oquendo. He presently life in Arizona and his mothers and fathers live nearby. Interviewing is what she does. What he truly enjoys doing is to perform handball but he is having difficulties to discover time for it.<br><br>Look into my webpage [http://Www.Tobynabors.com/ActivityFeed/MyProfile/tabid/61/userId/46195/Default.aspx Www.Tobynabors.com]

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en>AnomieBOT: Dating maintenance tags: {{Dn}}

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