**October 25**

**Ali Enayat**,
University of Gothenburg

**Tightness, solidity, and internal categoricity**

Inspired by a certain result about PA in Albert Visser's paper 'Categories of theories and interpretations', I introduced the notions of tightness and solidity (of an arbitrary theory) in my paper 'Variations on a Visserian theme'; using them Visser's result can be expressed as: PA is a solid theory (it is easy to show that solidity implies tightness). My aforementioned paper demonstrates that besides PA, certain other canonical theories such as Z_2 (Second Order Arithmetic), ZF, and KM (Kelley-Morse Class Theory) are also solid. The first talk in this series will present : (a) the proofs of solidity of PA and Z_2, and (b) the relationship between Väänänen's notion of internal categoricity with the notions of solidity and tightness. The second part will concentrate on establishing the failure of solidity/tightness of certain subtheories of PA and Z_2, including any subtheory of PA or Z_2 that is finitely axiomatizable.