On the number of variables in undecidable superintuitionistic propositional calculi

In this article, we construct an undecidable superintuitionistic propositional calculus using 3-variable axioms, i.e. an undecidable finitely axiomatizable extension of the intuitionistic propositional calculus with axioms containing only 3 variables. Since superintuitionistic propositional calculi cannot be axiomatized using less than 3 variables, this is the minimal possible number of variables.