Filtered category

In category theory, filtered categories generalize the notion of directed set understood as a category (hence called a directed category; while some use directed category as a synonym for a filtered category).

A category J is filtered when

A diagram is said to be of cardinality \kappa if the morphism set of its domain is of cardinality \kappa. A category J is filtered if and only if there is a cone over any finite diagram d: D\to J; more generally, for a regular cardinal \kappa, a category J is said to be \kappa-filtered if for every diagram d in J of cardinality smaller than \kappa there is a cone over d.

A filtered colimit is a colimit of a functor F:J\to C where J is a filtered category. This readily generalizes to \kappa-filtered limits. An ind-object in a category C is a presheaf of sets C^{op}\to Set which is a small filtered colimit of representable presheaves. Ind-objects in a category C form a full subcategory Ind(C) in the category of functors C^{op}\to Set. The category Pro(C)=Ind(C^{op})^{op} of pro-objects in C is the opposite of the category of ind-objects in the opposite category C^{op}.

Cofiltered categories

There is a dual notion of cofiltered category. A category J is cofiltered if the opposite category J^{\mathrm{op}} is filtered. In detail, a category is cofiltered when

A cofiltered limit is a limit of a functor F:J \to C where J is a cofiltered category.

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