In mathematics, specifically in category theory, an exponential object or map object is the categorical generalization of a function space in set theory. Categories with all finite products and exponential objects are called cartesian closed categories.
Let be a category with binary products and let and be objects of . An object together with a morphism is an exponential object if for any object and morphism there is a unique morphism (called the transpose of ) such that the following diagram commutes:
If exists for all objects in , then the functor defined on objects by and on arrows by , is a right adjoint to the product functor . For this reason, the morphisms and are sometimes called exponential adjoints of one another.
In the category of vector spaces, the exponential object always exists. As a familiar object of linear algebra, it is often written , the set of linear maps from to . This can be seen to have a vector space structure through a canonical isomorphism with the vector space of tensors .
In the category of topological spaces, the exponential object exists provided that is a locally compact Hausdorff space. In that case, the space is the set of all continuous functions from to together with the compact-open topology. The evaluation map is the same as in the category of sets. If is not locally compact Hausdorff, the exponential object may not exist (the space still exists, but it may fail to be an exponential object since the evaluation function need not be continuous). For this reason the category of topological spaces fails to be cartesian closed. However, the category of locally compact topological spaces is not cartesian closed either, since need not be locally compact for locally compact spaces and . A cartesian closed category of spaces is for example given by the full subcategory spanned by the compactly generated Hausdorff spaces.
In functional programming languages, the morphism is often called , and the syntax is often written . The morphism here must not to be confused with the eval function in some programming languages, which evaluates quoted expressions.
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