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On the complexity of propositional quantification in intuitionistic logic. (English) Zbl 0887.03002
The propositionally quantified intuitionistic logic H$$\pi+$$ is defined to be the set of all valid formulas, quantifiers being interpreted as ranging over all propositions. The main result of the paper says that H$$\pi+$$ is rather non-constructive: it is recursively isomorphic to full second-order logic. The autor suggests to restrict the class of underlying model structures and to take into consideration only those of them that are trees; in this way one could obtain another plausible intuitionistic propositional logic with quantifiers. The paper is not self-contained. In fact, only 1-reducibility of second-order logic to H$$\pi+$$ is actually proved, and the proof heavily relies on an earlier result (credited to Rabin and Scott) which allows to deal with a fragment of second-order logic admitting only certain binary relations. The converse (that second-order logic is 1-reducible to H$$\pi+$$) can be shown by methods used by the author in an earlier paper [J. Symb. Log. 58, 334-349 (1993; Zbl 0786.03016)].

##### MSC:
 03B20 Subsystems of classical logic (including intuitionistic logic)
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