Robert E. Kirk

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It was claimed by MCKAY [l] that the set of all finite binary trees is a characteristic set of Kripke frames for the intuitionistic propositional logic (I). This claim, however, was shown to be false by GABBAY and DEJONGH [2] who exhibited an I-invalid formula which holds in any structure based on a frame consisting of a finite binary tree. It is shown in(More)
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