Category of functors

This article focuses on the category of contravariant functors between two categories.

The category of functors

Let Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle C} and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle D} be two categories. The category of functors Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Funct(C^{op},Sets)} has

1. Objects are functors $F:C^{op}\to D$
2. A morphism of functors $F,G$ is a natural transformations $\eta :F\to G$; i.e., for each object $U$ of $C$, a morphism in $D$ $\eta _{U}:F(U)\to G(U)$ such that for all morphisms $f:U\to V$ in $C^{op}$, the diagram

commutes.

A natural isomorphism is a natural tranformation $\eta$ such that $\eta _{U}$ is an isomorphism in $D$ for every object $U$. One can verify that natural isomorphisms are indeed isomorphisms in the category of functors.

Examples

1. In the theory of schemes, the presheaves $h_{X}$ are often referred to as the functor of points of the scheme X. Yoneda's lemma allows one to think of a scheme as a functor in some sense, which becomes a powerful interpretation; indeed, meaningful geometric concepts manifest themselves naturally in this language, including (for example) functorial characterizations of smooth morphisms of schemes.

The Yoneda lemma

Let $C$ be a category and let $X,X'$ be objects of $C$. Then

1. If $F$ is any contravariant functor $F:C^{op}\to Sets$, then the natural transformations of $Mor_{C}(-,X)$ to $F$ are in correspondence with the elements of the set $F(X)$.
2. If the functors $Mor_{C}(-,X)$ and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Mor_C(-,X')} are isomorphic, then Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle X} and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle X'} are isomorphic in Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle C} . More generally, the functor Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle h:C\to Funct(C^{op},Sets)} , Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle X\mapsto h_X} , is an equivalence of categories between Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle C} and the full subcategory of representable functors in Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Funct(C^{op},Sets)} .

References

• David Eisenbud; Joe Harris (1998). The Geometry of Schemes. Springer-Verlag. ISBN 0-387-98637-5.