Herbrand's theorem is a fundamental result of mathematical logic obtained by Jacques Herbrand. It essentially allows a certain kind of reduction of first-order logic to propositional logic. Although Herbrand originally proved his theorem for arbitrary formulas of first-order logic, the simpler version shown here, restricted to formulas in prenex form containing only existential quantifiers, became more popular.
Statement
Let be a formula of first-order logic with quantifier-free, though it may contain additional free variables. This version of Herbrand's theorem states that the above formula is valid if and only ifthere exists a finite sequence of terms, possibly in an expansion of the language, with such that is valid. If it is valid, it is called a Herbrand disjunction for Informally: a formula in prenex form containing only existential quantifiers is provable in first-order logic if and only if a disjunction composed of substitution instances of the quantifier-free subformula of is a tautology. The restriction to formulas in prenex form containing only existential quantifiers does not limit the generality of the theorem, because formulas can be converted to prenex form and their universal quantifiers can be removed by Herbrandization. Conversion to prenex form can be avoided, if structural Herbrandization is performed. Herbrandization can be avoided by imposing additional restrictions on the variable dependencies allowed in the Herbrand disjunction.
Proof sketch
A proof of the non-trivial direction of the theorem can be constructed according to the following steps:
Starting from above downwards, remove the inferences that introduce existential quantifiers.
Remove contraction-inferences on previously existentially quantified formulas, since the formulas might not be identical anymore after the removal of the quantifier inferences.
The removal of contractions accumulates all the relevant substitution instances of in the right side of the sequent, thus resulting in a proof of, from which the Herbrand disjunction can be obtained.
However, sequent calculus and cut-elimination were not known at the time of Herbrand's theorem, and Herbrand had to prove his theorem in a more complicated way.
Herbrand disjunctions and expansion-tree proofs have been extended with a notion of cut. Due to the complexity of cut-elimination, Herbrand disjunctions with cuts can be non-elementarily smaller than a standard Herbrand disjunction.
Herbrand disjunctions have been generalized to Herbrand sequents, allowing Herbrand's theorem to be stated for sequents: "a Skolemized sequent is derivable iff it has a Herbrand sequent".
