Extendible formulas in two variables in intuitionistic logic
Nick Bezhanishvili, Dick de Jongh
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 Ghilardiâ€™s theorem that every extendible formula
is projective.