Category of elements

Category of elements

In category theory, for every presheaf P inhat C := mathbf{Set}^{C^{op the category of elements mathbf{El}(P) of P is the category defined as follows:
* its objects are pairs (A,a) where A is an object of C and ain P(A),
* its morphisms (A,a) o (B,b) are the morphisms f:A o B of C such that a=P(f)(b).

This construction can be extended into a functor mathbf{El} from hat C to mathbf{Cat}, the category of small categories.


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