Derivator

One of the motivating reasons for considering derivators is the lack of functoriality with the cone construction with triangulated categories. Derivators are able to solve this problem, and solve the inclusion of general homotopy colimits, by keeping track of all possible diagrams in a category with weak equivalences and their relations between each other. Heuristically, given the diagram\bullet \to \bulletwhich is a category with two objects and one non-identity arrow, and a functorF:(\bullet \to \bullet) \to Ato a category A with a class of weak-equivalences W (and satisfying the right hypotheses), we should have an associated functorC(F): \bullet \to A[W^{-1}]where the target object is unique up to weak equivalence in \mathcal{C}[W^{-1}]. Derivators are able to encode this kind of information and provide a diagram calculus to use in derived categories and homotopy theory. == Definition ==

Prederivators Formally, a prederivator \mathbb{D} is a 2-functor\mathbb{D}: \text{Ind}^{op} \to \text{CAT}from a suitable 2-category of indices to the category of categories. Typically such 2-functors come from considering the categories \underline{\text{Hom}}(I^{op}, A) where A is called the category of coefficients. For example, \text{Ind} could be the category of small categories which are filtered, whose objects can be thought of as the indexing sets for a filtered colimit. Then, given a morphism of diagramsf:I \to Jdenote f^* byf^*:\mathbb{D}(J) \to \mathbb{D}(I)This is called the inverse image functor. In the motivating example, this is just precompositition, so given a functor F_I \in \underline{\text{Hom}}(I^{op}, A) there is an associated functor F_J = F_I \circ f. Note these 2-functors could be taken to be\underline{\text{Hom}}(-,A[W^{-1}])where W is a suitable class of weak equivalences in a category A. Indexing categories There are a number of examples of indexing categories which can be used in this construction • The 2-category \text{FinCat} of finite categories, so the objects are categories whose collection of objects are finite sets. • The ordinal category \Delta can be categorified into a two category, where the objects are categories with one object, and the functors come from the arrows in the ordinal category. • Another option is to just use the category of small categories. • In addition, associated to any topological space X is a category \text{Open}(X) which could be used as the indexing category. • Moreover, the sites underlying the Zariski, Etale, etc., topoi of (X)_\tau for some scheme or algebraic space X along with their morphisms can be used for the indexing category • This can be generalized to any topos T, so the indexing category is the underlying site. Derivators Derivators are then the axiomatization of prederivators which come equipped with adjoint functors :f^? \dashv f_! \dashv f^* \dashv f_* \dashv f^! where f_! is left adjoint to f^* and so on. Heuristically, f_* should correspond to inverse limits, f_! to colimits. == References ==