1. Aldo Ursini (1985). Decision Problems for Classes of Diagonalizable Algebras. Studia Logica 44 (1):87 - 89.
    We make use of a Theorem of Burris-McKenzie to prove that the only decidable variety of diagonalizable algebras is that defined by 0=1. Any variety containing an algebra in which 01 is hereditarily undecidable. Moreover, any variety of intuitionistic diagonalizable algebras is undecidable.
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  2. Aldo Ursini (1979). A Modal Calculus Analogous to K4w, Based on Intuitionistic Propositional Logic, Iℴ. Studia Logica 38 (3):297 - 311.
    This paper treats a kind of a modal logic based on the intuitionistic propositional logic which arose from the provability predicate in the first order arithmetic. The semantics of this calculus is presented in both a relational and an algebraic way.Completeness theorems, existence of a characteristic model and of a characteristic frame, properties of FMP and FFP and decidability are proved.
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  3. Aldo Ursini (1978). On the Set of 'Meaningful' Sentences of Arithmetic. Studia Logica 37 (3):237 - 241.
    I give several characterizations of the set V₀ proposed in [3] as the set of meaningful and true sentences of first order arthimetic, and show that in Peano arithmetic the Σ₂ completeness of V₀ is provable.
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