The uniqueness of the fixed-point in every diagonalizable algebra
Studia Logica 35 (4):335 - 343 (1976)
| Abstract | It is well known that, in Peano arithmetic, there exists a formula Theor (x) which numerates the set of theorems. By Gödel's and Löb's results, we have that Theor (˹p˺) ≡ p implies p is a theorem ∼Theor (˹p˺) ≡ p implies p is provably equivalent to Theor (˹0 = 1˺). Therefore, the considered "equations" admit, up to provable equivalence, only one solution. In this paper we prove (Corollary 1) that, in general, if P (x) is an arbitrary formula built from Theor (x), then the fixed-point of P (x) (which exists by the diagonalization lemma) is unique up to provable equivalence. This result is settled referring to the concept of diagonalizable algebra (see Introduction) | |||||||||
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Roberto Magari (1975). Representation and Duality Theory for Diagonalizable Algebras. Studia Logica 34 (4):305 - 313.
Claudio Bernardi & Andrea Sorbi (1983). Classifying Positive Equivalence Relations. Journal of Symbolic Logic 48 (3):529-538.
V. Yu Shavrukov (1991). The Lindenbaum Fixed Point Algebra is Undecidable. Studia Logica 50 (1):143 - 147.
Aldo Ursini (1985). Decision Problems for Classes of Diagonalizable Algebras. Studia Logica 44 (1):87 - 89.
V. Yu Shavrukov (1997). Undecidability in Diagonalizable Algebras. Journal of Symbolic Logic 62 (1):79-116.
Claudio Bernardi (1975). The Fixed-Point Theorem for Diagonalizable Algebras. Studia Logica 34 (3):239 - 251.
Franco Montagna (1980). Interpretations of the First-Order Theory of Diagonalizable Algebras in Peano Arithmetic. Studia Logica 39 (4):347 - 354.
Giovanni Sambin (1976). An Effective Fixed-Point Theorem in Intuitionistic Diagonalizable Algebras. Studia Logica 35 (4):345 - 361.
Franco Montagna (1975). For Everyn, Then-Freely Generated Algebra is Not Functionally Free in the Equational Class of Diagonalizable Algebras. Studia Logica 34 (4):315 - 319.
Glaudio Bernardi (1975). On the Equational Class of Diagonalizable Algebras. Studia Logica 34 (4):321 - 331.
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