The notion of enriched category is a generalization of the notion of category.
Very often instead of merely having a set of morphisms from one object to another, a category will have a vector space of morphisms, or a topological space of morphisms, or some other such thing. This suggests that we should take the definition of (locally small) category and generalize it by replacing the hom-sets by hom-objects , which are objects in a suitable category . This gives the concept of ‘enriched category’.
The category must be monoidal, so that we can define composition as a morphism
So, a category enriched over (also called a category enriched in , or simply a -category), say , has a collection of objects and for each pair , a ‘hom-object’
We then mimic the usual definition of category.
We may similarly define a functor enriched over and a natural transformation enriched over , obtaining a strict 2-category of -enriched categories. By general 2-category theory, we thereby obtain notions of -enriched adjunction, -enriched equivalence, and so on.
See also enriched category theory.
Ordinarily enriched categories have been considered as enriched over a monoidal category. This is discussed in the section
More generally, one may think of a monoidal category as a bicategory with a single object and this way regard enrichment in a monoidal category as the special case of enrichment in a bicategory . This is discussed in the section
Let be a monoidal category with
tensor product ;
tensor unit ;
left unitor ;
right unitor .
A (small) -category (or -enriched category or category enriched over/in ) is
a set – called the set of objects;
for each ordered pair of objects in an object – called the hom-object or object of morphisms from to ;
for each ordered triple of objects of a morphism in – called the composition morphism;
for each object a morphism – called the identity element
such the following diagrams commute:
for all :
this says that composition in is associative;
this says that composition is unital.
If is a monoidal category, then an alternative way of viewing a -category is as a set together with a (lax) monoidal functor of the form
where the codomain is identified with the monoidal category of spans on , i.e., the local hom-category in the bicategory of spans of sets. Given an -category under the ordinary definition, the corresponding monoidal functor takes an object of to the span
Under the composition law, we get a natural map
which gives the tensorial constraint for a monoidal functor; the identity law similarly gives the unit constraint.
Conversely, by using a Yoneda-style argument, such a monoidal functor structure on induces an -enrichment on , and the two notions are equivalent.
Alternatively, we can equivalently describe a -enriched category as precisely a bicontinuous lax monoidal functor of the form
since bicontinuous functors of the form are precisely those of the form for some function , at least if is Cauchy complete.
such that the associativity and unitality diagrams, as written above, commute. Viewing a monoidal category as a 1-object bicategory , the notion of enrichment in coincides with the notion of enrichment in the bicategory .
If , are sets which come equipped with enrichments in , then a -functor consists of a function such that , together with a function , satisfying the constraint , and satisfying equations expressing coherence with the composition and unit data , of and . (Diagram to be inserted, perhaps.)
It is also natural to generalize further to categories enriched in a (possibly weak) double category. Just like for a bicategory, if is a double category, then a -enriched category consists of a set together with
satisfying analogues of the associativity and unit conditions. Note that is is exactly the same as a category enriched in the horizontal bicategory of ; the vertical arrows of play no role in the definition. However, they do play a role when it comes to define functors between -enriched categories. Namely, if and are -enriched categories, then a -functor consists of:
satisfying suitable equations. If is vertically discrete, i.e. just a bicategory with no nonidentity vertical arrows, then this is just the same as a -functor as defined above. However, for many this notion of functor is more general and natural.
Every -enriched category has an underlying ordinary category, usually denoted , defined by where is the unit object of .
If has a left adjoint (taking a set to the tensor or copower , viz. the coproduct of an -indexed set of copies of ), then any ordinary category can be regarded as enriched in by forming the composite
More generally, any (lax) monoidal functor between monoidal categories can be regarded as a “change of base”. By applying to its hom-objects, any category enriched over gives rise to one enriched over , and this forms a 2-functor from -Cat to -Cat, and in fact from -Prof to -Prof; see profunctor and 2-category equipped with proarrows.
Moreover, this operation is itself functorial from to . In particular, any monoidal adjunction gives rise to a 2-adjunction (and also for profunctors). The adjunction described above is a special case of this arising from the adjunction .
The idea of enriched categories is not unrelated to that of internal categories, but is different. One difference is that in a -enriched category, the objects still form a set (or a proper class) while the arrows are replaced by objects of , while in a category internal to , both the set of objects and the set of arrows are replaced by objects of .
Another difference is that for -enriched categories, can be any monoidal category, while for -internal categories, it must have pullbacks, which can be thought of as a generalization of cartesian monoidal structure. In particular, a -internal category with one object (that is, whose object-of-objects is a terminal object) is a monoid in with respect to the cartesian product, whereas a one-object -enriched category is a monoid in with respect to whatever monoidal structure we use to define enriched categories.
Nevertheless, internalization and enrichment are related in several ways. On the one hand, internal categories and enriched categories are both instances of monads in bicategories (the bicategory of spans and the bicategory of matrices, respectively). On the other hand, when is an -extensive category, such as Set or simplicial sets (or more generally any Grothendieck topos), (small) -enriched categories can be identified with -internal categories whose object-of-objects is discrete (that is, a coproduct of copies of the terminal object).
The page here mentions internalization and enrichment being the result of applying two different interpretation techniques to the same theory.
Beware: the term ‘simplicial category’ is also used to mean a category internal to simplicial sets. In fact, a category enriched in simplicial sets is a special case of a category internal to simplicial sets, namely one where the simplicial set of objects is discrete.
Again beware: the term ‘topological category’ is perhaps more commonly used to mean a category internal to Top. People also use it for topological concrete category. And again: a category enriched in Top is a special case of one internal to Top, namely one where the space of objects is discrete.
An algebroid, or linear category, is a category enriched over Vect. Here is the category of vector spaces over some fixed field , equipped with its usual tensor product. It is common to emphasize the dependence on and call a category enriched over Vect a -linear category.
A (Lawvere) metric space is a category enriched over the poset of extended positive real numbers, where is .
An ultrametric space is a category enriched over the poset of extended positive real numbers, where is .
A torsor over some group may be modeled by a category enriched over the discrete category on the set , where is the group operation. Not every such category determines a torsor, however; it must be nonempty as well as Cauchy complete.
A standard reference on enriched categories is
Vista of some modern generalizations is in
Change of base is discussed in
John Armstrong: Enriched categories.