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On theorems of Gödel and Kreisel: Completeness and Markov’s principle. (English) Zbl 0801.03038
Summary: In 1957, Gödel proved that completeness for intuitionistic predicate logic HPL implies forms of Markov’s Principle, MP. The result first appeared, with G. Kreisel’s refinements and elaborations, in J. Symb. Log. 27, 139-158 (1963; Zbl 0117.010). Featuring large in the Gödel-Kreisel proofs are applications of the axiom of dependent choice, DC. Also in play is a form of Herbrand’s theorem, one allowing a reduction of HPL derivations for negated prenex formulae to derivations of negations of conjunctions of suitable instances. First, we here show how to deduce Gödel’s results by alternative means, ones arguably more elementary than those of Kreisel [loc. cit.]. We avoid DC and Herbarnd’s theorem by marshalling simple facts about negative translations and Markov’s rule. Second, the theorems of Gödel and Kreisel are commonly interpreted as demonstrating the unprovability of completeness of HPL, if means of proof are confined within strictly intuitionistic metamathematics. In the closing section, we assay some doubts about such interpretations.

##### MSC:
 03F50 Metamathematics of constructive systems
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##### References:
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