In elementary algebra (or more generally in a ring), multiplication distributes over addition, meaning that
and dually. Since multiplication and addition are a decategorification of products and coproducts in a category, we can categorify this relationship to say what it means for products to distribute over coproducts in any category. More generally, we can talk about arbitrary limits distributing over colimits. Note that this is not the same as limits commuting with colimits, although in some cases they are related.
Given small categories $I$ and $K$, for any diagram $D\colon I\times K\to C$ in a category $C$ that admits all necessary limits and colimits we have a morphism
induced using the universal property of $colim_K$ and $lim_I$ by the family of composites
of the limit projection for $i\in I$ and the colimit injection for $k\in K$. Recall that $I$-limits are said to commute with $K$-colimits if and only if $f$ is an isomorphism for any $D$.
Now we note that $f$ factors as a composition
Here $K^I$ is the functor category and $D'\colon I\times K^I\to C$ is obtained from $D$ by precomposition with the functor $(\pi_1,ev) : I\times K^I \to I\times K$ that sends a pair $(i,s)$ to $(i,s(i))$. The map $g$ is induced by the diagonal functor $K\to K^I$ sending each $k$ to the corresponding constant functor, while $h$ is defined by the universal properties of $colim_{K^I}$ and $lim_I$ from the family of composites
of the limit projection for $i\in I$ and the colimit injection for $c(i)\in K$. We say that $I$-limits distribute over $K$-colimits if $h$ is an isomorphism for any $D$.
More generally, we can allow $K$ to vary with $i$, becoming a functor $K\colon I\to Cat$. Now instead of $K^I$ we use the limit category $lim_I K$, and the diagram $D$ is defined on the Grothendieck construction (lax colimit) $\int^I K$. We recover the above definition when $K$ is a constant functor.
If $\mathcal{K}$ is a class of small categories rather than a single one, we say that $I$-limits distribute over $\mathcal{K}$-colimits in a category $C$ if the corresponding map is an isomorphism for any $K:I\to Cat$ taking values in $\mathcal{K}$. For instance, if $\mathcal{K}$ is the class of all discrete categories, we obtain the notion of $I$-limits distributing over coproducts, and so on. If instead $I$-limits distribute over $K$-colimits for any single $K\in \mathcal{K}$, we may say instead that “$I$-limits distribute over uniform $K$-limits.”
I don’t know whether distributivity over uniform colimits is actually any weaker, but if it is, then the non-uniform version seems more correct.
Yet more generally, let $u : I\to J$ be any Grothendieck fibration and $v : K\to I$ a Grothendieck opfibration. Since fibrations are exponentiable functors, we can form the distributivity pullback
Thus $Y$ is the dependent product $\Pi_I K$, and $X = I\times_J \Pi_I K$, while the functor $p:X\to K$ is “evaluation”. Now in any sufficiently complete category, there is a Beck-Chevalley? isomorphism
This is because any pullback of a fibration is an exact square. This isomorphism has a mate
and we say that right Kan extensions along $u$ distribute over left Kan extensions along $v$ if this mate is an isomorphism.
If $J=1$ then this reduces to the above non-uniform notion, where $K\to I$ is the Grothendieck construction of a functor $I\to Cat$. Moreover, since isomorphisms of $J$-diagrams are pointwise, right and left Kan extensions along fibrations and opfibrations respectively are given by fiberwise limits and colimits respectively, and dependent exponentials are preserved by pullback (the Beck-Chevalley condition), we can say that right Kan extensions along $u$ distribute over left Kan extensions along $v$ if and only if the above non-uniform distributivity condition holds for the fibers of $v$ over each object of $J$.
The formulation in terms of Kan extensions generalizes naturally to derivators, and can also be internalized to internal categories and fibrations in any locally cartesian closed category.
(See Section 6 in ABLR.)
Given a sound doctrine $D$ of limits, we have the associated notions of $D$-limits, $D$-filtered colimits, and $D$-filtered cocompletion $D-Ind(C)$ of a category $C$ together with the associated colimit functor $colim\colon D-Ind(C)\to C$.
We say that some class of limits distributes over $D$-filtered colimits if the functor colim preserves these limits.
See Appendix A in AR for a comparison of this definition to the above explicit definition.
Observe that distributivity is asymmetric with respect to limits and colimits, whereas commutativity of limits and colimits is a symmetric notion. In some cases, however, distributivity and commutativity are equivalent. This happens exactly when the above functor $g$ is an isomorphism, which is to say that the diagonal map $K\to K^I$ is a final functor.
Thus, for instance, finite limits distribute over (uniform) filtered colimits if and only if finite limits commute with filtered colimits. The same is true for finite products and sifted colimits.
The distributivity of finite products over arbitrary coproducts is the most classical version. See distributivity for monoidal structures for various generalizations.
More generally, finite products distribute over arbitrary colimits in any cartesian closed category, such as Set.
If $D$ is the doctrine of finite limits, then $D$-filtered colimits are precisely filtered colimits and the $D$-filtered cocompletion of $C$ is $Ind(C)$, the category of Ind-objects in $C$.
According to Definition 5.11 in ALR, a category is precontinuous if it admits small limits and filtered colimits, and small limits distribute over filtered colimits, i.e., the functor $colim: Ind(C)\to C$ is continuous.
In particular, any locally finitely presentable category, equivalently the category of algebras over some finitary essentially algebraic theory, is precontinuous. See Theorem 5.13 and Lemma 5.14 in ALR.
In fact, as shown in ALR, precontinuous categories with continuous functors that preserve filtered colimits form the accessible equational hull? of the bicategory of locally finitely presentable categories and continuous functors that preserve filtered colimits.
If $D$ is the doctrine of finite products, then $D$-filtered colimits are precisely sifted colimits and the $D$-filtered cocompletion of $C$ is $Sind(C)$, the nonabelian derived category? of $C$.
According to Definition 4.5 in ALRalg, a category is algebraically exact if it admits small limits and sifted colimits, and small limits distribute over sifted colimits, i.e., the functor $colim: Sind(C)\to C$ is continuous. In particular (Example 5.3 in ALRalg), any variety of algebras is an algebraically exact category. In particular, this includes Set.
In fact, as shown in ALRalg, algebraically exact categories with continuous functors that preserve sifted colimits form the accessible equational hull? of the bicategory of varieties of algebras and continuous functors that preserve sifted colimits.
When the definition for Kan extensions is internalized to internal categories and fibrations, and then specialized to the case of discrete categories, the notion of distributivity of dependent products over dependent sums encoded by a distributivity pullback becomes the statement that in the codomain fibration, indexed products? distribute over indexed coproducts? in this sense.
As a nontrivial example of the $K\colon I\to Cat$ case, let $I$ be the walking cospan $0 \to 1 \leftarrow 2$, and let $K(0) = J$ for some category $J$ while $K(1)=K(2)=\ast$, the terminal category. Then $\int^I K$ is $J$ together with a new terminal object $1$ and a new object $2$ having only a map to $1$, and so a diagram $D: \int^I K \to C$ consists of a $J$-diagram $D_0$ in a slice category $C/Z$ over some object $Z$ along with a morphism $f:Z\to Y$. The limit category $\lim_I K$ is isomorphic to $J$, and the functor $D'\colon I\times \lim_I K\to C$ has $D'(0,j) = D_0(j)$, with $D'(1,j) =Z$ and $D'(2,j)=Y$. Therefore, $\colim_{\lim_I K} \lim_I D'$ is the $J$-colimit of the pullback of $D_0$ along $f$, while $\lim_{i\in I} \colim_{K(i)} D$ is the pullback along $f$ of the colimit of $D_0$. Thus, the distributivity condition for this $K$ holds if and only if $J$-colimits are universal (stable under pullback).
Jiří Adámek, Jiří Rosický, Enrico Vitale, Algebraic theories. A categorical introduction to general algebra CUP 2011.
Jiří Adámek, William Lawvere, Jiří Rosický, Continuous Categories Revisited TAC 11, 2003.
Jiří Adámek, William Lawvere, Jiří Rosický, How algebraic is algebra TAC 8, 2001.
Jiří Adámek, Francis Borceux, Stephen Lack, Jiří Rosický, A classification of accessible categories JPAA 175, 2002.
Jiří Adámek, Jiří Rosický, Algebra and local presentability: how algebraic are they? (A survey).
Last revised on January 15, 2018 at 17:59:42. See the history of this page for a list of all contributions to it.