Bipolar spaces
  Some basic features of the simultaneous inclusion of discrete fibrations and discrete opfibrations on a category A in the category of categories over A are studied; in particular, the reflections and the coreflections of the latter in the former are considered, along with a negation-complement operator which, applied to a discrete fibration, gives a functor with values in discrete opfibrations (and vice versa) and which turns out to be classical, in that the strong contraposition law holds. Such an analysis is developed in an appropriate conceptual frame that encompasses similar "bipolar" situations and in which a key role is played by "cofigures", that is components of products; e.g. the classicity of the negation-complement operator corresponds to the fact that discrete opfibrations (or in general "closed parts") are properly analyzed by cofigures with shape in discrete fibrations ("open parts"), that is, that the latter are "coadequate" for the former, and vice versa. In this context, a very natural definition of "atom" is proposed and it is shown that, in the above situation, the category of atoms reflections is the Cauchy completion of A. 
Claudio Pisani