Category Theory/(Co)cones and (co)limits
Definition (cone):
Let be a category and let be a diagram in . A cone over is an object of , together with morphisms for each , so that for each (so that ) we have
 .
Definition (limit):
Let be a category, let be a diagram in and let
Definition (diagonal):
Let be a category, and let be an object of . Further, suppose that the object and the morphisms and constitute a product of with itself in . Then the diagonal (also called "diagonal morphism") is the unique morphism
such that .
Definition (antidiagonal):
Let be a category, and let be an object of . Further, suppose that the object and the morphisms and constitute a coproduct of with itself in . Then the antidiagonal (also called "antidiagonal morphism") is the unique morphism
such that .
Exercises edit

 Let be a category such that every two objects of have a product. Suppose further that is another category, and that is a functor. Let be an object of . Use the universal property of the product in order to show that there exists a functor that sends an object of to the object of .
 Prove that any morphism in gives rise to a natural transformation .
 Can we weaken the assumption that every two objects of have a product? (Hint: Consider the image of the class function on objects associated to the functor .)