Functor category

 ( Functors ) 

In category theory, a branch of mathematics, a functor category $D^C$ is a category where the objects are the $F:\; C\; \backslash to\; D$ and the are natural transformations $\backslash eta:\; F\; \backslash to\; G$ between the functors (here, $G:\; C\; \backslash to\; D$ is another object in the category). Functor categories are of interest for two main reasons:

• many commonly occurring categories are (disguised) functor categories, so any statement proved for general functor categories is widely applicable;
• every category embeds in a functor category (via the Yoneda embedding); the functor category often has nicer properties than the original category, allowing certain operations that were not available in the original setting.

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Definition Suppose $C$ is a small category (i.e. the objects and morphisms form a set rather than a proper class) and $D$ is an arbitrary category. The category of functors from $C$ to $D$, written as Fun($C$, $D$), Funct($C$,$D$), $C,D$, or $D\; ^C$, has as objects the covariant functors from $C$ to $D$, and as morphisms the natural transformations between such functors. Note that natural transformations can be composed: if $\backslash mu\; (X)\; :\; F(X)\; \backslash to\; G(X)$ is a natural transformation from the functor $F\; :\; C\; \backslash to\; D$ to the functor $G\; :\; C\; \backslash to\; D$, and $\backslash eta(X)\; :\; G(X)\; \backslash to\; H(X)$ is a natural transformation from the functor $G$ to the functor $H$, then the composition $\backslash eta(X)\backslash mu(X)\; :\; F(X)\; \backslash to\; H(X)$ defines a natural transformation from $F$ to $H$. With this composition of natural transformations (known as vertical composition, see natural transformation), $D^C$ satisfies the axioms of a category.

In a completely analogous way, one can also consider the category of all contravariant functors from $C$ to $D$; we write this as Funct($C^\backslash text\{op\},D$).

If $C$ and $D$ are both preadditive categories (i.e. their morphism sets are and the composition of morphisms is bilinear), then we can consider the category of all from $C$ to $D$, denoted by Add($C$,$D$).

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Examples

• If $I$ is a small discrete category (i.e. its only morphisms are the identity morphisms), then a functor from $I$ to $C$ essentially consists of a family of objects of $C$, indexed by $I$; the functor category $C\; ^I$ can be identified with the corresponding product category: its elements are families of objects in $C$ and its morphisms are families of morphisms in $C$.

• An arrow category $\backslash mathcal\{C\}^\backslash rightarrow$ (whose objects are the morphisms of $\backslash mathcal\{C\}$, and whose morphisms are commuting squares in $\backslash mathcal\{C\}$) is just $\backslash mathcal\{C\}^\backslash mathbf\{2\}$, where 2 is the category with two objects and their identity morphisms as well as an arrow from one object to the other (but not another arrow back the other way).
• A graph theory consists of a set of arrows and a set of vertices, and two functions from the arrow set to the vertex set, specifying each arrow's start and end vertex. The category of all directed graphs is thus nothing but the functor category $\backslash textbf\{Set\}^C$, where $C$ is the category with two objects connected by two parallel morphisms (source and target), and Set denotes the category of sets.
• Any group $G$ can be considered as a one-object category in which every morphism is invertible. The category of all $G$-sets is the same as the functor category Set^$G$. Natural transformations are $G$-maps.
• Similar to the previous example, the category of K-linear representations of the group $G$ is the same as the functor category Vect _K^$G$ (where Vect _K denotes the category of all over the field K).
• Any ring $R$ can be considered as a one-object preadditive category; the category of left modules over $R$ is the same as the additive functor category Add($R$,$\backslash textbf\{Ab\}$) (where $\backslash textbf\{Ab\}$ denotes the category of abelian groups), and the category of right $R$-modules is Add($R^\backslash text\{op\}$,$\backslash textbf\{Ab\}$). Because of this example, for any preadditive category $C$, the category Add($C$,$\backslash textbf\{Ab\}$) is sometimes called the "category of left modules over $C$" and Add($C^\backslash text\{op\}$,$\backslash textbf\{Ab\}$) is the "category of right modules over $C$".
• The category of presheaf on a topological space $X$ is a functor category: we turn the topological space into a category $C$ having the open sets in $X$ as objects and a single morphism from $U$ to $V$ if and only if $U$ is contained in $V$. The category of presheaves of sets (abelian groups, rings) on $X$ is then the same as the category of contravariant functors from $C$ to $\backslash textbf\{Set\}$ (or $\backslash textbf\{Ab\}$ or $\backslash textbf\{Ring\}$). Because of this example, the category Funct($C^\backslash text\{op\}$, $\backslash textbf\{Set\}$) is sometimes called the "category of presheaves of sets on $C$" even for general categories $C$ not arising from a topological space. To define sheaves on a general category $C$, one needs more structure: a Grothendieck topology on $C$. (Some authors refer to categories that are equivalent to $\backslash textbf\{Set\}^C$ as presheaf categories.)

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Facts Most constructions that can be carried out in $D$ can also be carried out in $D^C$ by performing them "componentwise", separately for each object in $C$. For instance, if any two objects $X$ and $Y$ in $D$ have a product $X\backslash times\; Y$, then any two functors $F$ and $G$ in $D^C$ have a product $F\backslash times\; G$, defined by $(F\; \backslash times\; G)(c)\; =\; F(c)\backslash times\; G(c)$ for every object $c$ in $C$. Similarly, if $\backslash eta\_c\; :\; F(c)\; \backslash to\; G(c)$ is a natural transformation and each $\backslash eta\_c$ has a kernel $K\_c$ in the category $D$, then the kernel of $\backslash eta$ in the functor category $D^C$ is the functor $K$ with $K(c)\; =\; K\_c$ for every object $c$ in $C$.

As a consequence we have the general rule of thumb that the functor category $D^C$ shares most of the "nice" properties of $D$:

• if $D$ is complete (or cocomplete), then so is $D^C$;
• if $D$ is an abelian category, then so is $D^C$;

We also have:

• if $C$ is any small category, then the category $\backslash textbf\{Set\}^C$ of presheaves is a topos.

So from the above examples, we can conclude right away that the categories of directed graphs, $G$-sets and presheaves on a topological space are all complete and cocomplete topoi, and that the categories of representations of $G$, modules over the ring $R$, and presheaves of abelian groups on a topological space $X$ are all abelian, complete and cocomplete.

The embedding of the category $C$ in a functor category that was mentioned earlier uses the Yoneda lemma as its main tool. For every object $X$ of $C$, let $\backslash text\{Hom\}(-,X)$ be the contravariant representable functor from $C$ to $\backslash textbf\{Set\}$. The Yoneda lemma states that the assignment

$X\; \backslash mapsto\; \backslash operatorname\{Hom\}(-,X)$

is a full embedding of the category $C$ into the category Funct($C^\backslash text\{op\}$,$\backslash textbf\{Set\}$). So $C$ naturally sits inside a topos.

The same can be carried out for any preadditive category $C$: Yoneda then yields a full embedding of $C$ into the functor category Add($C^\backslash text\{op\}$,$\backslash textbf\{Ab\}$). So $C$ naturally sits inside an abelian category.

The intuition mentioned above (that constructions that can be carried out in $D$ can be "lifted" to $D^C$) can be made precise in several ways; the most succinct formulation uses the language of adjoint functors. Every functor $F\; :\; D\; \backslash to\; E$ induces a functor $F^C\; :\; D^C\; \backslash to\; E^C$ (by composition with $F$). If $F$ and $G$ is a pair of adjoint functors, then $F^C$ and $G^C$ is also a pair of adjoint functors.

The functor category $D^C$ has all the formal properties of an exponential object; in particular the functors from $E\; \backslash times\; C\; \backslash to\; D$ stand in a natural one-to-one correspondence with the functors from $E$ to $D^C$. The category $\backslash textbf\{Cat\}$ of all small categories with functors as morphisms is therefore a cartesian closed category.

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See also

• Diagram (category theory)

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