For the formal definition, consider a covariant functor F : J -> C. A limit of F is an object L of C, together with morphisms φX : L -> F(X) for every object X of J, such that for every morphism f : X -> Y in J, we have F(f) φX = φY, and such that the following universal property is satisfied:
If F has a limit (which it need not), then the limit is defined up to a unique isomorphism, and is denoted by lim F.
If J is a small category and every functor from J to C has a limit, then the limit operation forms a functor from the functor category (see category theory) CJ to C. For example, if J is a discrete category and C is the category Ab of abelian groups, then lim : AbJ -> Ab is the functor which assigns to every family of abelian groups its direct product. More generally, if J is a small category arising from a partially ordered set, then lim: AbJ -> Ab assigns to every system of abelian groups its inverse limit.
The limit functor lim : CJ -> C (if it exists) has as left adjoint the diagonal functor C -> CJ which assigns to every object N of C the constant functor whose value is always N on objects and idN on morphisms.
If every functor from every small category J to C has a limit, then the category C is called complete. Many important categories are complete: groups, abelian groups, sets, modules over some ring, topological spaces and compact Hausdorff spaces. In general, a category C is complete iff it contains arbitrary products, and equalizers[?] for any pair of parallel morphisms.
A functor G : C -> D is continuous if it maps limits to limits, in the following sense: whenever I is a small category and a functor F : I -> C has limit L together with morphisms φX : L -> F(X), then the functor GF : I -> D has limit G(L) with maps G(φX). Important examples of continuous functors are given by the representable ones: if U is some object of C, then the functor GU : C -> Set with GU(V) = MorD(U, V) for all objects V in D is continuous.
The importance of adjoint functors lies in the fact that every functor which has a left adjoint (and therefore is a right adjoint) is continuous. In the category Ab of abelian groups, this for example shows that the kernel of a product of homomorphisms is naturally identified with the product of the kernels. Also, limit functors themselves are continuous.
Colimits are defined analogous to limits: A colimit of the functor F : J -> C is an object L of C, together with morphisms φX : F(X) -> L for every object X of J, such that for every morphism f : X -> Y in J, we have φY F(f) = φX, and such that the following universal property is satisfied:
The colimit of F, unique up to unique isomorphism if it exists, is denoted by colim F.
Limits and colimits are related as follows: A functor F : J -> C has a colimit if and only if for every object N of C, the functor X |-> MorC(F(X),N) (which is a covariant functor on the dual category Jop) has a limit. If that is the case, then
The category C is called cocomplete if every functor F : J -> C with small J has a colimit. The following categories are cocomplete: sets, groups, abelian groups, modules over some ring and topological spaces.
A covariant functor G : C -> D is cocontinuous if it transforms colimits into colimits. Every functor which has a right adjoint (and is a left adjoint) is cocontinuous. As an example in the category Grp of groups: the functor F : Set -> Grp which assigns to every set S the free group over S has a right adjoint (the forgetfull functor Grp -> Set) and is therefore cocontinuous. The free product[?] of groups is an example of a colimit construction, and it follows that the free product of a family of free groups is free.
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