Back and forth between algebraic geometry, algebraic logic, sheaves and forcing

Abstract

We study sheaves in the context of a duality theory for lattice structure endowed with extra operations, and in the context of forcing in a topos. Using Sheaf duality theory of Comer for cylindric algebras, we give a representation theorem of of distributive bounded lattices expanded by modalities (functions distributing over joins) as the continuous sections of sheaves. Our representation is defined via a contravariant functor from an algebraic category to a category of sheaves. We show that if our category is a small site (cartesian closed with a stability condition on pullbacks), then we can define a notion of forcing using this category. In particular, we define fuzzy forcing by interpreting the additional Lukasiewicz conjunction $\otimes$ as induced by a tensor product in the target monodial category of pre-sheaves. We also study topoi as semantics for higher order logic of many sorted theories in connection to set theory, and the quasi-topoi based on $MV$ algebras, for fuzzy logic. We show that the interpretation of a theory $T$, in this case into $Set_{\Omega}$ where $\Omega$ is an almost sub-object classifier in a quasi-topos $CAT$ defined from $T$, is completed by defining semantics for $\otimes$, and this is done similarly to its defining clause in forcing. We give applications to many-valued logics and various modifications of first order logic and multi-modal logic, set in an algebraic framework.