category theory

# Filtered categories

## Idea

A filtered category is a categorification of the concept of directed set. In addition to having an upper bound (but not necessarily a coproduct) for every pair of objects, there must also be an upper bound (but not necessarily a coequaliser) for every pair of parallel morphisms.

A diagram $F:D\to C$ where $D$ is a filtered category is called a filtered diagram. A colimit of a filtered diagram is called a filtered colimit.

A category whose opposite is filtered is called cofiltered.

## Definitions

###### Definition

A (finitely) filtered category is a category $C$ in which every finite diagram has a cocone.

That is, for any finite category $D$ and any functor $F:D\to C$, there exists an object $c\in C$ and a natural transformation $F\to \Delta c$ where $\Delta c:D\to C$ is the constant diagram at $c$.

Equivalently, filtered categories can be characterized as those categories where, for every finite diagram $J$, the diagonal functor $\Delta :C\to {C}^{J}$ is final. This point of view can be generalized to other kinds of categories whose colimits are well-behaved with respect to a type of limit, such as sifted categories.

This can be rephrased in more elementary terms by saying that:

• There exists an object of $C$ (the case when $D=\varnothing$)
• For any two objects ${c}_{1},{c}_{2}\in C$, there exists an object ${c}_{3}\in C$ and morphisms ${c}_{1}\to {c}_{3}$ and ${c}_{2}\to {c}_{3}$.
• For any two parallel morphisms $f,g:{c}_{1}\to {c}_{2}$ in $C$, there exists a morphism $h:{c}_{2}\to {c}_{3}$ such that $hf=hg$.

Just as all finite colimits can be constructed from initial objects, binary coproducts, and coequalizers, so a cocone on any finite diagram can be constructed from these three.

In constructive mathematics, the elementary rephrasing above is equivalent to every Bishop-finite diagram admitting a cocone.

### Higher filteredness

More generally, if $\kappa$ is an infinite regular cardinal (or an arity class), then a $\kappa$-filtered category is one such that any diagram $D\to C$ has a cocone where $D$ has $<\kappa$ arrows. The usual filtered categories are then the case $\kappa =\omega$. Note that a preorder is $\kappa$-filtered as a category just when it is $\kappa$-directed as a preorder.

### Generalized filteredness

Even more generally, if $𝒥$ is a class of small categories, a category $C$ is called $𝒥$-filtered if $C$-colimits commute with $𝒥$-limits in Set. When $𝒥$ is the class of all $\kappa$-small categories for an infinite regular cardinal $\kappa$, then $𝒥$-filteredness is the same as $\kappa$-filteredness as defined above. See ABLR.

If $𝒥$ is the class consisting of the terminal category and the empty category — which is to say, the class of $\kappa$-small categories when $\kappa$ is the finite regular cardinal $2$ — then being $𝒥$-filtered in this sense is equivalent to being connected. Note that this is not what the explicit definition given above for infinite regular cardinals would specialize to by simply setting $\kappa =2$ (that would be simply inhabitation).

## References

• Jiri Adámek, Francis Borceux, Stephen Lack, and Jiri Rosický, “A classification of accessible categories”, Journal of Pure and Applied Algebra 175:7-30, 2002, web page with PS fulltext.

Revised on March 19, 2013 14:35:53 by Ingo Blechschmidt (137.250.162.16)