nLab
filtered limit

Idea

A filtered colimit is a suitable category such as Set is a colimit of shape such that it commutes with all finite limits.

Definition

Definition

A filtered colimit or finitely filtered colimit is a colimit of a functor $F:D\to C$ where $D$ is a filtered category.

For $\kappa$ a regular cardinal a $\kappa$-filtered colimit is one over a $\kappa$-filtered category.

Similarly, a cofiltered limit is a limit of a functor $F:D\to C$ where $D$ is a cofiltered category, or equivalently of a contravariant functor $F:D\to C$ (that is a functor $F:D^{\mathrm{op}}\to C$) where $D$ is a filtered category.

Remark

A cofiltered limit may also be called a filtered limit (although this can be unclear); the respective terms filtered direct limit and filtered inverse limit are also popular.

A functor that preserves all finitely filtered colimits is called a finitary functor .

Properties

General

Remark

For $C$ and $D$ two diagram categories and

$$F:C\times D\to \mathrm{Set}$$F : C \times D \to Set

a diagram, there is a canonical morphism

$$\lambda :{\underrightarrow{\lim }}_C{\underleftarrow{\lim }}_{D}F\to {\underleftarrow{\lim }}_D{\underrightarrow{\lim }}_{C}F$$\lambda : {\lim_\to}_C {\lim_\leftarrow}_D F \to {\lim_\leftarrow}_D {\lim_\to}_C F

from the colimit over $C$ of the limit over $D$ to the limit over $D$ of the colimit over $C$ of $F$:

$\lambda$ is given by a cone, whose components

$${\lambda }_d:{\underrightarrow{\lim }}_C{\underleftarrow{\lim }}_{D}F\to {\underrightarrow{\lim }}_{C}F(-,d)$$\lambda_d : {\lim_\to}_C {\lim_\leftarrow}_D F \to {\lim_\to}_C F(-,d)

are in turn given by a cocone with components

$$({\lambda }_d{)}_c:{\underleftarrow{\lim }}_{D}F(c,-)\to {\underrightarrow{\lim }}_{C}F(-,d).$$(\lambda_d)_c : {\lim_\leftarrow}_D F(c,-) \to {\lim_\to}_C F(-,d) \,.

This finally take to have as components

$${\underleftarrow{\lim }}_{D}F(c,d)\to F(c,d)\to {\underrightarrow{\lim }}_{C}F(c,d).$${\lim_\leftarrow}_D F(c,d) \to F(c,d) \to {\lim_\to}_C F(c,d) \,.

One checks that this indeed makes all the components be natural and makes the origina morphism exist.

Notice that in general $\lambda$ is not an isomorphism.

Definition

We say the limit ${{\lim }_{\leftarrow }}_{D}F(-,-)$ commutes? with the colimit ${{\lim }_{\to }}_{C}F(-,-)$ is the morphism $\lambda$ above is an isomorphism

$${\underrightarrow{\lim }}_C{\underleftarrow{\lim }}_{D}F\stackrel{\simeq }{\to }{\underleftarrow{\lim }}_D{\underrightarrow{\lim }}_{C}F.$${\lim_\to}_C {\lim_\leftarrow}_D F \stackrel{\simeq}{\to} {\lim_\leftarrow}_D {\lim_\to}_C F \,.

Propositon

In Set, filtered colimits commute with finite limits.

In fact, filtered categories $C$ are precisely those shapes of diagram categories such that colimits over them commute with all finite limits.

A detailed components proof of the first part is in Borceux, theorem I2.13.4.

For more on this see also limits and colimits by example.

According to 1.5 and 1.21 in LPAC, a category has $\kappa$-directed colimits precisely if it has $\kappa$-filtered ones, and a functor preserves $\kappa$-directed colimits iff it preserves $\kappa$-filtered ones. A proof of this result, following Adamek & Rosicky, may be found here?.

The fact that directed colimits suffice to obtain all filtered ones may be regarded as a convenience similar to the fact that all colimits can be constructed from coproducts and coequalizers. Of course, a dual result holds for codirected limits.

Flat functors and points of presheaf toposes

Let $C$ be a small category. A functor $F:C\to \mathrm{Set}$ is flat if it is a filtered colimit of representable functors.

Equivalently, a module $F:C\to \mathrm{Set}$ is flat if and only if the tensor product

$$-{\otimes }_{C}F:{\mathrm{Set}}^{C^{\mathrm{op}}}\to \mathrm{Set}$$- \otimes_C F: Set^{C^{op}} \to Set

is left exact. One may prove as a corollary that if $C$ is finitely complete, $F$ is flat if and only if it is left exact (preserves finite limits). Since this tensor product is automatically a left adjoint, we have the following basic result:

Proposition

For $C$ a small category, the category of topos points of the presheaf topos ${\mathrm{Set}}^{C^{\mathrm{op}}}$ (i.e., geometric morphisms $\mathrm{Set}\to {\mathrm{Set}}^{C^{\mathrm{op}}}$ and natural transformations between them) is equivalent to the category of flat modules on $C$.

Description in Set, Grp, Top and alike

Elements in filtered colimits in Set and Grp are given as classes of equivalences, so called germs. Filtered limits in Set and Top are given as families of compatible elements, so called threads.

More

(More was/is to be written here, including an application to geometric realization, relation to Diaconescu's theorem, and perhaps more.)

Applications

Filtered colimits are also important in the theory of locally presentable and accessible categories. See also pro-object and ind-object.

Related concepts

• filtered (∞,1)-colimit

References

Section 2.13 in part I of

• Francis Borceux, Handbook of Categorical Algebra