# Solovay's completeness theorem without fixed points

**Date**: Wednesday 15 November 2017, 14:00 – 15:00**Location**: Mathematics Level 8, MALL 2, School of Mathematics**Type**: Proofs, constructions and computations seminars, Pure Mathematics seminars, Seminar series**Cost**: Free

#### Fedor Pakhomov, Steklov Mathematical Institute Moscow. Part of the PCC seminar series

Abstract: The main result I will talk about will be a new proof of Solovay's theorem on arithmetical completeness of Gödel-Löb provability logic GL. Originally, completeness of GL with respect to the interpretation of Box as provability in PA was proved by R. Solovay in 1976. The key part of Solovay's proof was his construction of an arithmetical evaluation of a given modal formula that made the formula unprovable 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 has 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. Also I will use a similar technique to give an example of a natural Orey sentence for PA.