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Solovay’s completeness without fixed points
In this paper we present a new proof of Solovay’s theorem on arithmetical completeness of Gödel-Löb provability logic GL. Originally, completeness of GL with respect to interpretation of □ as provability in PA was proved by Solovay in 1976. The key part of Solovay’s proof was his construction of an arithmetical evaluation for a given modal formula that made the formula unprovable in PA if it were unprovable in GL. The arithmetical sentences for the evaluations were constructed using certain arithmetical fixed points. The method developed by Solovay have been used for establishing similar semantics for many other logics. In our proof we develop new more explicit construction of required evaluations that doesn’t use any fixed points in their definitions. To our knowledge, it is the first alternative proof of the theorem that is essentially different from Solovay’s proof in this key part.