Cartesian closed category: Encyclopedia II - Cartesian closed category - Examples

Cartesian closed category - Examples

Examples of cartesian closed categories include:

• The category Set of all sets, with functions as morphisms, is cartesian closed. The product X×Y is the cartesian product of X and Y, and Z^Y is the set of all functions from Y to Z. The adjointness is expressed by the following fact: the function f : X×Y → Z is naturally identified with the function g : X → Z^Y defined by g(x)(y) = f(x,y) for all x in X and y in Y.
• The category of finite sets, with functions as morphisms, is cartesian closed for the same reason.
• If G is a group, then the category of all G-sets is cartesian closed. If Y and Z are two G-sets, then Z^Y is the G-set of equivariant maps from Y to Z with trivial G action (an G-equivariant map from Y to Z is by definition an application f : Y → Z such that g.(f(y)) = f(g.y) for every g in G and y in Y).
• The category of finite G-sets is also cartesian closed.
• The category Cat of all small categories (with functors as morphisms) is cartesian closed; the exponential C^D is given by the functor category consisting of all functors from D to C, with natural transformations as morphisms.
• If C is a small category, then the functor category Set^C consisting of all covariant functors from C into the category of sets, with natural transformations as morphisms, is cartesian closed. If F and G are two functors from C to Set, then the exponential F^G is the functor whose value on the object X of C is given by the set of all natural transformations from (X,−) × G to F.
  • The earlier example of G-sets can be seen as a special case of functor categories: every group can be considered as a one-object category, and G-sets are nothing but functors from this category to Set
  • The category of all directed graphs is cartesian closed; this is a functor category as explained under functor category.
• In algebraic topology, cartesian closed categories are particularly easy to work with, and it is regrettable that neither the category of topological spaces with continuous maps nor the category of smooth manifolds with smooth maps is cartesian closed. Substitute categories have therefore been considered: the category of compactly generated Hausdorff spaces is cartesian closed, as is the category of Frölicher spaces.
• If X is a topological space, then the open sets in X form the objects of a category O(X) for which there's a unique morphism from U to V if U is a subset of V and no morphism otherwise. This category is cartesian closed; the "product" of U and V is the intersection of U and V and the exponential U^V is the interior of U∪(X\V).

The following categories are not cartesian closed:

• The category of all vector spaces over some fixed field is not cartesian closed, neither is the category of all finite-dimensional vector spaces. While they have products (called direct sums), the product functors don't have right adjoints.
• The category of abelian groups is not cartesian closed, for the same reason.

Adapted from the Wikipedia article "Examples", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki