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 $ (T_0,T,\Lambda) $ (which we call a spatial Coste context), each $ T_0 $-model can be associated with a $ T $-modelled space and that this yields a dual adjunction between the category of $ T_0 $-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 However, most of his proofs remain unpublished.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 $ T_0 $-models and give another proof of Coste adjunction. Moreover, we also extend spectra of $ T_0 $-models to relative spectra of $ T_0 $-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.