Here I’ll try to reorganize the entry limit according to the structure discussed in the forum entry “objects” and tools to compute them. As this page will be complete I’ll move it into the Lab. Editing of this page is strongly encouraged :)
In category theory a limit of a diagram $F\colon D \to C$ in a category $C$ is an object $lim F$ of $C$ equipped with morphisms to the objects $F(d)$ for all $d \in D$, such that everything in sight commutes. Moreover, the limit $lim F$ is the universal object with this property, i.e. the “most optimized solution” to the problem of finding such an object.
One can think of the category of cones over $F\colon D\to C$ as the collection of all displacements of arrows stemming from a fixed “source” $C$ (the base of the cone) suitably linked by morphisms.
This in turn can be regarded as a functor from the diagram $[0]\star D$ to $C$ ‘extending’ $F$ (see the page over quasi-category); now the universal property makes $\text{lim}\; F$ a terminal object in the category of these functors.
In the $(\infty,1)$-categorical setting, this suggests to define limits again in terms of terminal objects, but using the joins of quasi-categories instead of the join of categories?.
For $K$ and $C$ two quasi-categories and $F : K \to C$ a morphism of quasi-categories (i.e. nothing more than a map of simplicial sets?. Then the limit of $F$ is, if it exists, the quasi-categorical terminal object in the over quasi-category $C_{/F}$:
where $C_{/F}=Hom_{F}([0] \star K, C )$. (This is HTT Definition 1.2.13.4)
As an object of $C_{/F}$ the limit of $F$ can be regarded as a 0-simplex in the simplicial set $n\mapsto Hom_{F}([n] \star K, C )\subset C^{[n] \star K}$: see HTT Notation 1.2.8.4 and the join of quasi-categories? page.
Fosco: this generalization is not at all evident, since the definition of weighted limit seems to conflict (see the page weighted join) with the classical one.
There is an evident generalization to weighted limit?s: replace in the above the join $[0] \star K$ with the weighted join $[0] \star_W K$ where $W$ is any functor $W \colon D \to Set$ – then called the weight. The $W$-weighted limit of $F$, $\lim_W F$, also written $\{W,F\}$, is, if it exists, the quasicategorical terminal oject in $Hom_{F}([0] \star_W K, C )$.
Below this line is the entry limit as it was on the nLab on January 4, 2010.
In category theory? a limit of a diagram? $F : D \to C$ in a category? $C$ is an object? $lim F$ of $C$ equipped with morphisms to the objects $F(d)$ for all $d \in D$, such that everything in sight commutes. Moreover, the limit $lim F$ 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 functor?s as a classifying space for maps into a diagram. So in some sense the limit object $lim F$ “subsumes” the entire diagram $F(D)$ 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?.
Often, the general theory of limits (but not colimits!) works better if the source of $F$ is taken to be the opposite category? $D^op$ (or equivalently, if $F$ 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 defintion of adjoint functor?s (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? $F : D^{op} \to C$ and since the functor comes equipped with the information about what its domain is, one can just write $\lim F$ for its limit. But often it is helpful to indicate how the functor is evaluated on objects, in which case the limit is written $\lim_{d \in D} F(d)$; this is used particularly when $F$ 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 (repsectively) 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 $lim_{\leftarrow}$ and $lim_{\rightarrow}$ are used instead of $lim$ and $colim$. (Actually, the arrows should be directly underneath the ‘$lim$’s, something like ${\lim \atop \longleftarrow}$ and ${\lim \atop \longrightarrow}$. But the text should also be at normal size.)
Confusingly, many authors restrict the meanings of these alternative terms to (co)limits whose sources are directed set?s; 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.
Let in the following $D$ be a small category? and Set? the category of sets (possibly realized as the category $U Set$ of $U$-small sets with respect to a given Grothendieck universe?.)
The limit of a Set-valued functor $F : D^{op} \to Set$ is the hom-set?
in the functor category? $[D^{op}, Set]$ (the presheaf? category), where
is the functor constant on the point?, i.e. the terminal? diagram.
The set $lim F$ is equivalently called
the set of global sections of $F$;
the set of generalized element?s of $F$.
The set $lim F$ can be equivalently expressed as an equalizer? of a product?, explicitly:
In particular, the limit of a set-valued functor always exists.
Notice the important triviality that the covariant hom-functor? comutes with set-valued limits: for every set $S$ we have a bijection of sets
where $Hom(S, F(-)) : D^{op} \to Set$.
The above formula generalizes straightforwardly to
a notion of limit for functors $F : D^{op} \to C$ for $C$ an arbitrary category if we take the object “$lim F$” to be a presheaf? on $C$. The true $lim F$ is then, if it exists, the object of $C$ representing? this presheaf.
More precisely, using the the Yoneda embedding? $Y : C \to [C^{op}, Set]$ define for $F : D^{op} \to C$ the presheaf? $\hat \lim F \in [C^{op}, Set]$ by the analog of the above formula
for all $d \in D$.
Here the $\lim$ 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:
So also the presheaf?-valued limit always exist. If this presheaf is representble? by an object $lim F$ of $F$, then this is the limit of $F$:
In the above formulation, there is an evident generalization to weighted limit?s:
replace in the above the constant terminal functor $pt : D^{op} \to Set$ with any functor $W : D^{op} \to Set$ – then called the weight –, then the $W$-weighted limit of $F$
often written
is, if it exists, the object representing the presheaf
i.e. such that
naturally in $c \in C$.
The very definition of limit as above asserts that the covariant hom-functor? $Hom(c,-) : C \to Set$ commutes with forming limits. Indeed, the definition is equivalent to saying that the hom-functor? is a continuous functor?.
Unwrapping the above abstract definition of limits yields the following more hands-on description in terms of universal cones.
Let $F : D^{op} \to C$ be a functor.
Notice that for every object $c \in C$ an element
is to be identified with a collection of morphisms
for all $d \in D$, such that all triangles
commute. Such a collection of morphisms is called a cone over $F$, for the obvious reason.
If the limit $\lim F \in C$ of $F$ exist, then it singles out a special cone given by the composite morphism
where the first morphism picks the identity morphism? on $\lim F$ and the second one is the defining bijection of a limit as above.
This cone
is called the universal cone over $F$, because, again by the defining proprty of limit as above, everey other cone $\{c \to F(d)\}_{d \in D}$ as above is bijectively related to a morphism $c \to \lim F$
By inspection one finds that, indeed, the morphism $c \to \lim F$ is the morphism which exhibits the factorization of the cone $\{c \to F(d)\}_{d \in D}$ through the universal limit cone
An illustrative example is the following: a limit of the identity functor? $Id_c:C\to C$ is, if it exists, an initial object? of $C$.
Given categories $D$ and $C$, limits over functors $D^{op} \to C$ 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.
For $D$ a small category? and $C$ any category, the functor category? $[D^{op},C]$ is the category of $D$-diagram?s in $C$. Pullback along the functor $D^{op} \to pt$ to the terminal? category $pt = \{\bullet\}$ induces a functor
which sends every object of $C$ to the diagram functor constant on this object.
The left adjoint?
of this functor is, if it exists, the functor which sends every diagram to its colimit? and the right adjoint? is, if it exists, the functor
which sends every diagram to its limit. The Hom-isomorphisms of these adjunction?s state precisely the universal property of limit and colimit? given above.
Concretely this means that for all $c \in C$ we have a bijection
Compare this with the discussion at Kan extension?.
From this perspective, a limit is a special case of a Kan extension?, as described there, namely a Kan extension to the point?.
The definition of a limit as a terminal cone has a straightforward generalization to the context of (infinity,1)-categories?.
For $K$ and $C$ two quasi-categories? and $F : K \to C$ a morphism of quasi-categories?, the limit over $F$ is, if it exists, the quasi-categorical terminal object? in the over quasi-categories $C_{/F}$:
For more details see limit in quasi-categories?.
Here are some important examples of limits, classified by the shape of the diagram:
For $F : D^{op} \to Set$ any functor and $const_{*} : D^{op} \to Set$ the functor constant on the point?, the limit of $F$ is the hom-set?
in the functor category?, i.e. the set of natural transformation?s from the constant functor into $F$.
For $C$ a locally small? category, for $F : D^{op} \to C$ a functor and writing $C(c, F(-)) : CD^{op} \to Set$, we have
Depending on how one introduces limits this holds by definition or is an easy consequence.
Let $D$ be a small category and let $D'$ be any category. Let $C$ be a category which admits limits of shape $D$. Write $[D',C]$ for the functor category?. Then * $[D',C]$ admits $D$-shaped limits; * these limits are computed objectwise (“pointwise”) in $C$: for $F : D^{op} \to [D',C]$ a functor we have for all $d' \in D'$ that $(lim F)(d') \simeq lim (F(-)(d'))$. Here the limit on the right is in $C$.
Let $D$ and $D'$ be small catgeories and let $C$ be a category which admits limits of shape $D$ as well as limits of shape $D'$. Then these limits commute with each other, in that
for $F : D^{op} \times {D'}^{op} \to C$ a functor , with corresponding induced functors $F_D : {D'}^{op} \to [D^{op},C]$ and $F_{D'} : {D}^{op} \to [{D'}^{op},C]$, then
Let $R : C \to C'$ be a functor that is right adjoint? to some functor $L : C' \to C$. Let $D$ be a small category such that $C$ admits limits of shape $D$. Then $R$ commutes with $D$-shaped limits in $C$ in that
for $F : D^{op} \to C$ some diagram, we have
Using the adjunction isomorphism and the above fact that Hom commutes with limits, one obtains for every $c' \in C'$
Since this holds naturally for every $c'$, the Yoneda lemma, corollary II? on uniquenes of representing objects implies that $R (lim F) \simeq lim (G \circ F)$.
The limit of $F : D^{op} \to C$ is, if it exists, a subobject of the product of the $F(d)$, namely the equalizer of
and
In particular therefore, a category has all limits already if it has all products and equalizers.
See limits and colimits by example? for what this formula says for instance for the special case $C =$ Set?.
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?.