Limits, colimits, and spectra of modelled spaces

Abstract

It is well-known that the construction of Zariski spectra of (commutative) rings yields a dual adjunction between the category of rings and the category of locally ringed spaces. There are many constructions of spectra of algebras in various contexts giving such adjunctions. Michel Coste unified them in the language of categorical logic by showing that, for an appropriate triple (T0,T,Λ) (which we call a spatial Coste context), each T0-model can be associated with a T-modelled space and that this yields a dual adjunction between the category of T0-models and the category of T-modelled spaces and “admissible” morphisms. However, most of his proofs remain unpublished. In this paper, we introduce an alternative construction of spectra of T0-models, which is reminiscent of Zariski spectra, and give a new proof of Coste adjunction. Moreover, we also extend spectra of T0-models to relative spectra of T0-modelled spaces and prove the existence of limits and colimits in the involved categories of modelled spaces. We can deduce, for instance, that the category of ringed spaces whose stalks are fields is complete and cocomplete.