The Visser rules form a basis of admissibility for the intuitionistic propositional calculus. We show how one can characterize the existence of covers in certain models by means of formulae. Through this characterization, we provide a new proof of the admissibility of a weak form of the Visser rules. Finally, we use this observation, coupled with a description of a generalization of the disjunction property, to provide a basis of admissibility for the intermediate logics 𝖡𝖣2 and GSc.
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About Jeroen Goudsmit
Jeroen is a mathematician working on compliance in the financial sector, focussing on quantitative integrity risk management.