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For example, the initial object in any concrete category with free objects will be the free object generated by the empty set (since the free functor, being left adjoint to the forgetful functor to Set, preserves colimits). Initial and terminal objects may also be characterized in terms of universal properties and adjoint functors.
Let C be a category with finite products and a terminal object 1. A list object over an object A of C is: an object L A, a morphism o A : 1 → L A, and; a morphism s A : A × L A → L A; such that for any object B of C with maps b : 1 → B and t : A × B → B, there exists a unique f : L A → B such that the following diagram commutes:
Examples of limits and colimits in Ring include: The ring of integers Z is an initial object in Ring. The zero ring is a terminal object in Ring. The product in Ring is given by the direct product of rings. This is just the cartesian product of the underlying sets with addition and multiplication defined component-wise.
The partially ordered class of all ordinal numbers is cocomplete but not complete (since it has no terminal object). A group, considered as a category with a single object, is complete if and only if it is trivial. A nontrivial group has pullbacks and pushouts, but not products, coproducts, equalizers, coequalizers, terminal objects, or initial ...
A groupoid object in the category of sets is precisely a groupoid in the usual sense: a category in which every morphism is an isomorphism.Indeed, given such a category C, take U to be the set of all objects in C, R the set of all morphisms in C, the five morphisms given by () =, =, (,) =, () = and () =.
If A is an object of C, then the functor from C to Set that sends X to Hom C (X,A) (the set of morphisms in C from X to A) is an example of such a functor. If C is a small category (i.e. the collection of its objects forms a set), then the contravariant functors from C to Set, together with natural transformations as morphisms, form a new ...
The diagonal functor : assigns to each object of the diagram , and to each morphism : in the natural transformation in (given for every object of by =). Thus, for example, in the case that J {\displaystyle {\mathcal {J}}} is a discrete category with two objects, the diagonal functor C → C × C {\displaystyle {\mathcal {C}}\rightarrow ...
Let T, η, μ be a monad over a category C.The Kleisli category of C is the category C T whose objects and morphisms are given by = (), (,) = (,).That is, every morphism f: X → T Y in C (with codomain TY) can also be regarded as a morphism in C T (but with codomain Y).