What is Limit (category Theory)?

In category theory, a branch of mathematics, the abstract notion of a limit captures the essential properties of universal constructions that are used in various parts of mathematics, like products and inverse limits. Accordingly, the dual notion of a colimit, generalizes disjoint unions and direct sums. Limits and colimits have strong relationships to the categorial concepts of universal morphisms and adjoint functors.

Definition

Before defining limits, it is useful to define the auxiliary notion of a cone of a functor. Consider two categories J and C and a functor F : J → C. A cone of F is an object N of C, together with a family of morphisms ψ_X : N → F(X), one for each object X of J, such that for every morphism f : X → Y in J, we have F(f) o ψ_X = ψ_Y.

A limit of a functor is just a universal cone. In detail, a cone (L, φ_X) of a functor F : J → C is a limit of that functor if and only if for any cone (N, ψ_X) of F, there exists precisely one morphism u : N → L such that φ_X o u = ψ_X for all X. We may say that the morphisms ψ_X factor through L with the unique factorization u. u is called the mediating morphism.



As for every universal property, this definition describes a balanced state of generality: The limit object L has to be general enough to allow any other cone to factor through it; on the other hand, L has to be sufficiently specific, so that only one such factorization is possible for every cone.

It is possible that a functor does not have a limit at all. However, if a functor has two limits then there exists a unique isomorphism between the respective limit objects which commutes with the respective cone maps; this isomorphism is given by the unique factorization from one limit to the other. Thus limits are unique up to isomorphism and can be denoted by lim F.

Examples

The definition of limits is general enough to subsume several constructions useful in practical settings. In the following we will consider the limit (L, φ_X) of a functor F : J → C.

• Terminal objects. If J is the empty category, then the above definitions imply that every object of C is a cone of F. The limit of F is any object that has a unique factorization through any other object. This is just the definition of a terminal object.
• Products. If J is a discrete category then the functor F is essentially nothing but a family of objects of C, indexed by J. The limit L of F is called the product of these objects. The special case where J consists of just two objects (which we will call 1 and 2) then defines a binary product. For example, assume that C is the category Set and let J be the discrete two-element category. The binary product L will then just be the cartesian product F(1) × F(2) in Set. The morphisms φ₁ and φ₂ are the projections to the respective components of the tuples from L.

  In many algebraic contexts, such as (abelian) groups, rings, Boolean algebras, etc., products are just direct products, where the operations are defined pointwise. Another well-known product is the product topology in Top, the category of topological spaces and continuous maps. If a partially ordered set is viewed as a category C, then products in C are greatest lower bounds while arbitrary cones are just lower bounds.

• Equalizers. If J is a two-object category with two parallel morphisms from object 1 to object 2 then the limit L is called an equalizer.
• Kernels. A kernel is just a special case of an equalizer where one of the morphisms is a zero morphism.
• Pullbacks. Let F be a functor that picks out three objects X, Y, and Z in C, where the only non-identity morphisms are f : X → Z and g : Y → Z. The limit L of F is called a pullback or a fiber product. It can nicely be visualized as a commutative square:

• Inverse limits. Let J be a directed poset (considered as a small category by adding arrows i → j if and only if i ≤ j) and let F : J → C be a contravariant functor. The limit of F is called (confusingly) an inverse limit or projective limit but also known as a directed limit.

All of the above examples follow a common scheme for the definition of limits: in order to model a limit construction, such as a product of sets, one uses a functor that "picks out" the relevant objects (and sometimes morphisms) from the category C. Consequently, the category J is usually a small category and has fewer elements than the category C. If one considers a finite category J then the above constructions can also be specified by giving the objects and morphisms that the functor F maps to. For example one may talk about an "equalizer of two morphisms" instead of calling this limit an "equalizer of a functor that maps the only two non-trivial morphisms in J to certain values".

However, J may well be a large category, i.e. one that has a proper class of objects. For example, the product of all sets exists and is just the empty set (indeed, this is the only possible cone on all families of sets that contain the empty set).

Complete categories

A category C is called complete if and only if every functor F : J → C, where J is any small category, has a limit; i.e. "all small limits in C exist". Similarly, if every such functor with J finite has a limit, then C is said to have finite limits.

Many important categories are complete: groups, abelian groups, sets, modules over some ring, topological spaces and compact Hausdorff spaces. Typical examples of categories that are not complete are categories with some "size restriction": the category of finite groups or the category of finite-dimensional vector spaces over a fixed field.

The Existence Theorem for Limits states that a category is complete if and only if it has equalizers and products over arbitrary sets of objects. Note that one doesn't require products of proper classes of objects to exist. It turns out that the property of having all (even large) limits is too strong to be practically relevant. Any category with this property necessarily is of a very restricted form: for any two objects there can be at most one morphism from one object to the other.

If J is a particular small category and every functor from J to C has a limit, then the limit operation forms a functor from the functor category C^J to C. For example, if J is a discrete category and C is the category Ab of abelian groups, then lim : Ab^J → Ab is the functor which assigns to every J-indexed family of abelian groups its direct product. More generally, if J is a small category arising from a partially ordered set, then lim: Ab^J → Ab assigns to every J indexed system of abelian groups its corresponding inverse limit.

Continuous functors

It is a natural question to ask, which functors are compatible with the construction of limits in the sense that they map limits to limits. These functors are called continuous or limit preserving. Formally, a functor G : C → D is continuous if and only if, for every small category I and every functor F : I → C that has a limit (L,φ_X) in C, the functor GF : I → D has the limit (G(L), G(φ_X) ). Since the Existence Theorem for Limits mentioned above shows that all limits can be expressed by products and equalizers, it is sufficient for continuity if G preserves these special limits.

Important examples of continuous functors are given by representable ones: if U is some object of C, then the functor G_U : C → Set with G_U(V) = Mor_C(U, V) for all objects V in C 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. This illustrates that one may also say that a continuous functor commutes with the formation of limits.

Being a universal construction, limits also have other strong relationships to adjoint functors. The limit functor lim : C^J → C (if it exists) has as left adjoint the diagonal functor C → C^J which assigns to every object N of C the constant functor whose value is always N on objects and id_N on morphisms. In particular, limit functors are continuous; intuitively, this means that the order in which two limits are taken doesn't matter.

Colimits

The dual notion of limits and cones are colimits and co-cones. A motivation for colimits has been given by Joseph Goguen in "A Categorical Manifesto":

Given a species of structure, say widgets, then the result of interconnecting a system of widgets to form a super-widget corresponds to taking the colimit of the diagram of widgets in which the morphisms show how they are interconnected.

Although it is straightforward to obtain the definitions of colimits and co-cones by inverting all morphisms in the above definitions, we will explicitly state them here:

Consider two categories J and C and a covariant functor F : J → C. A co-cone of F is an object N of C, together with a family of morphisms ψ_X : F(X) → N for every object X of J, such that for every morphism f : X → Y in J, we have ψ_Y o F(f)= ψ_X.

A colimit of a functor is a universal co-cone: a co-cone (L, φ_X) of a functor F : J → C is a colimit of F if and only if for any co-cone (N, ψ_X) of F, there exists precisely one morphism u : L → N such that u o φ_X = ψ_X for all X.



If it exists, the colimit of F is unique up to a unique isomorphism and is denoted by colim F.

Examples of colimits are given by the dual versions of the ones given above:

• Initial objects
• Coproducts
• Coequalizers
• Cokernels
• Pushouts
• Direct limits

The category C is called co-complete if every functor F : J → C with small J has a colimit. Similarly, a category is said to have finite colimits if every such functor with finite J has a colimit.

The following categories are co-complete: sets, groups, abelian groups, modules over some ring and topological spaces.

A covariant functor that commutes with the construction of colimits is said to be cocontinuous or colimit preserving. Every functor which has a right adjoint (and hence is a left adjoint) is cocontinuous.

As an example in the category of groups, Grp, the functor F : Set → Grp which assigns to every set S the free group over S has a right adjoint (the forgetful 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.

Limits and colimits are related as follows: If a functor F : J → C has a colimit then it is determined by the relation

Hom_C(colim F, N) = lim Hom_C(F–,N)

where N is any object of C and Hom_C(F–,N) is the functor from J to Set given by the composition of a Hom functor and F.

Creation of limits and colimits

Limits for a diagram D: J → X are said to be created by a functor F: X → Y if every limit cone of FD has a unique pre-image cone under F in X, and additionally this unique pre-image is a limit cone of D.

For example, the forgetful functor from Grp to Set creates limits, that is to say, for example that the product of groups is determined by their underlying sets. This is in a way characteristic of algebraic situations.

There is a dual notion for colimits. In the case of groups the aforementioned forgetful functor creates only filtered colimits.

Preservation of limits and colimits

A functor F: X → Y preserves limits if the image under F of every universal cone (indexed by a small category) of X is a universal cone of Y (cf. the section Continuous functors above). The dual notion is obtained by replacing everywhere `limit' by `colimit' and `universal cone' by `universal co-cone'. As direct limits are a particular case of colimits, an important subcase is preservation of direct limits. A functor F: X → Y preserves direct limits, if the image under F of every universal co-cone of X indexed by a directed poset is a universal co-cone of Y.

Interchange of limits

Theorem: Let I be a category with finitely many objects, and J a small filtered category. For any bifunctor

F: I × J → Set

there is a natural isomorphism

In words, filtered colimits in Set commute with finite limits, and (dually) cofiltered limits in Set commute with finite colimits.

Weak limits

A weak limit is defined like a limit, except that the uniqueness property of the mediating morphism is dropped. This specialises to weak pullbacks, weakly terminal objects etc.

References

• Mac Lane, Saunders (1998). Categories for the Working Mathematician, 2nd ed., New York: Springer. ISBN 0-387-98403-8.