Extendible Formulas in Two Variables in Intuitionistic Logic

Authors:
Nick Bezhanishvili;Dick Jongh
Affiliations:
Department of Computing, Imperial College London, London, UK SW7 2AZ;Institute for Logic, Language & Computation (ILLC), University of Amsterdam, Amsterdam, The Netherlands 1090 GE
Venue:
Studia Logica
Year:
2012

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Abstract

We give alternative characterizations of exact, extendible and projective formulas in intuitionistic propositional calculus IPC in terms of n-universal models. From these characterizations we derive a new syntactic description of all extendible formulas of IPC in two variables. For the formulas in two variables we also give an alternative proof of Ghilardi's theorem that every extendible formula is projective.