In category theory a limit of a diagram in a category is an object of equipped with morphisms to the objects for all , such that everything in sight commutes. Moreover, the limit is the universal object with this property, i.e. the “most optimized solution” to the problem of finding such an object.
The limit construction has a wealth of applications throughout category theory and mathematics in general. In practice, it is possibly best thought of in the context of representable functors as a classifying space for maps into a diagram. So in some sense the limit object “subsumes” the entire diagram into a single object, as far as morphisms into it are concerned. The corresponding universal object for morphisms out of the diagram is the colimit.
An intuitive general idea is that a limit of a diagram is the locus or solution set of a bunch of equations, where each of the coordinates is parametrized by one of the objects of the diagram, and where the equations are prescribed by the morphisms of the diagram. This idea is explained more formally here.
Often, the general theory of limits (but not colimits!) works better if the source of is taken to be the opposite category (or equivalently, if is taken to be a contravariant functor). This is what we do below. In any given situation, of course, you use whatever categories and functors you're interested in.
In some cases the category-theoretic notion of limit does reproduce notions of limit as known from analysis. See the examples below.
In correspondence to the local definition of adjoint functors (as discussed there), there is a local definition of limits (in terms of cones), that defines a limit (if it exists) for each individual diagram, and there is a global definition, which defines the limit for all diagrams (in terms of an adjoint).
If all limits over the given shape of diagrams exist in a category, then both definitions are equivalent.
See also the analogous discussion at homotopy limit.
A limit is taken over a functor and since the functor comes equipped with the information about what its domain is, one can just write for its limit. But often it is helpful to indicate how the functor is evaluated on objects, in which case the limit is written ; this is used particularly when is given by a formula (as with other notation with bound variables.)
In some schools of mathematics, limits are called projective limits, while colimits are called inductive limits. Also seen are (respectively) inverse limits and direct limits. Both these systems of terminology are alternatives to using ‘co-’ when distinguishing limits and colimits. The first system also appears in pro-object and ind-object.
Correspondingly, the symbols and are used instead of and .
Confusingly, many authors restrict the meanings of these alternative terms to (co)limits whose sources are directed sets; see directed limit. In fact, this is the original meaning; projective and inductive limits in this sense were studied in algebra before the general category-theoretic notion of (co)limit.
There is a general abstract definition of limits in terms of representable functors, which we describe now. This reproduces the more concrete and maybe more familiar description in terms of universal cones, which is described further below.
The limit of a Set-valued functor is the hom-set
The set is equivalently called
the set of global sections of ;
the set of generalized elements of .
In particular, the limit of a set-valued functor always exists.
Notice the important triviality that the covariant hom-functor commutes with set-valued limits: for every set we have a bijection of sets
The above formula generalizes straightforwardly to a notion of limit for functors for an arbitrary category if we construct a certain presheaf on which we will call . The actual limit is then, if it exists, the object of representing this presheaf.
for all .
Here the on the right is again that of Set-valued functors defined before.
By the above this can also be written as
or, suppressing the subscripts for readability:
In the above formulation, there is an evident generalization to weighted limits:
replace in the above the constant terminal functor with any functor – then called the weight –, then the -weighted limit of
is, if it exists, the object representing the presheaf
i.e. such that
naturally in .
Unwrapping the above abstract definition of limits yields the following more hands-on description in terms of universal cones.
Let be a functor.
Notice that for every object an element
is to be identified with a collection of morphisms
for all , such that all triangles
commute. Such a collection of morphisms is called a cone over , for the obvious reason.
If the limit of exist, then it singles out a special cone given by the composite morphism
where the first morphism picks the identity morphism on and the second one is the defining bijection of a limit as above.
is called the universal cone over , because, again by the defining property of limit as above, every other cone as above is bijectively related to a morphism
By inspection one finds that, indeed, the morphism is the morphism which exhibits the factorization of the cone through the universal limit cone
Given categories and , limits over functors may exist for some functors, but not for all. If it does exist for all functors, then the above local definition of limits is equivalent to the following global definition.
which sends every object of to the diagram functor constant on this object.
The left adjoint
Concretely this means that for all we have a bijection
The notion of limit, being fundamental to category theory, generalizes to many other situations. Examples include the following.
The central point about examples of limits is:
Categorical limits are ubiquitous.
To a fair extent, category theory is all about limits and the other universal constructions: Kan extensions, adjoint functors, representable functors, which are all special cases of limits – and limits are special cases of these.
Listing examples of limits in category theory is much like listing examples of integrals in analysis: one can and does fill books with these. (In fact, that analogy has more to it than meets the casual eye: see coend for more).
Keeping that in mind, we do list some special cases and special classes of examples that are useful to know. But any list is necessarily wildly incomplete.
Here are some important examples of limits, classified by the shape of the diagram:
Frequently some limits can be computed in terms of other limits. This makes things easier since we only have to assume that categories have, or functors preserve, some easier-to-verify class of limits in order to obtain results about a larger one.
The most common example of this is the computation of limits in terms of products and equalizers. Specifically, if the limit of and the products and all exist, then is a subobject of , namely the equalizer of
Conversely, if both of these products exist and so does the equalizer of this pair of maps, then that equalizer is a limit of . In particular, therefore, a category has all limits as soon as it has all products and equalizers, and a functor defined on such a category preserves all limits as soon as it preserves products and equalizers.
For a locally small category, for a functor and writing , we have
Depending on how one introduces limits this holds by definition or is an easy consequence.
Let be a small category and let be any category. Let be a category which admits limits of shape . Write for the functor category. Then * admits -shaped limits; * these limits are computed objectwise (“pointwise”) in : for a functor we have for all that . Here the limit on the right is in .
for some diagram, we have
Using the adjunction isomorphism and the above fact that Hom commutes with limits, one obtains for every
Since this holds naturally for every , the Yoneda lemma, corollary II on uniqueness of representing objects implies that .
Let and be small catgeories and let be a category which admits limits of shape as well as limits of shape . Then these limits commute with each other, in that
for a functor , with corresponding induced functors and , then
This follows from the above proposition and the characterization of the limit as right adjoint to the functor defined above in the section on adjoints.
In general limits do not commute with colimits. But under a number of special conditions of interest they do. More on that at commutativity of limits and colimits.