In , a branch of mathematics, a closed category is a special kind of .
In a , the external hom (x, y) maps a pair of objects to a set of s. So in the , this is an object of the category itself. In the same vein, in a closed category, the (object of) morphisms from one object to another can be seen as lying inside the category. This is the internal hom [x, y].
Every closed category has a forgetful functor to the category of sets, which in particular takes the internal hom to the external hom.
A closed category can be defined as a with a so-called internal Hom functor
with left Yoneda arrows
natural in and and dinatural in , and a fixed object of with a natural isomorphism
and a dinatural transformation
all satisfying certain coherence conditions.
are closed categories. In particular, any topos is closed. The canonical example is the .
are closed categories. The canonical example is the FdVect with finite-dimensional vector spaces as objects and linear maps as morphisms.
More generally, any is a closed category. In this case, the object is the monoidal unit.
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In mathematics, the Rel has the class of sets as and binary relations as . A morphism (or arrow) R : A → B in this category is a relation between the sets A and B, so R ⊆ A × B. The composition of two relations R: A → B and S: B → C is given by (a, c) ∈ S o R ⇔ for some b ∈ B, (a, b) ∈ R and (b, c) ∈ S. Rel has also been called the "category of correspondences of sets". The category Rel has the Set as a (wide) , where the arrow f : X → Y in Set corresponds to the relation F ⊆ X × Y defined by (x, y) ∈ F ⇔ f(x) = y.
In mathematics, especially in , a closed monoidal category (or a monoidal closed category) is a that is both a and a in such a way that the structures are compatible. A classic example is the , Set, where the monoidal product of sets and is the usual cartesian product , and the internal Hom is the set of functions from to . A non- example is the , K-Vect, over a field . Here the monoidal product is the usual tensor product of vector spaces, and the internal Hom is the vector space of linear maps from one vector space to another.