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.
A morphism in Rel is a relation, and the corresponding morphism in the to Rel has arrows reversed, so it is the converse relation. Thus Rel contains its opposite and is .
The involution represented by taking the converse relation provides the dagger to make Rel a .
The category has two functors into itself given by the hom functor: A binary relation R ⊆ A × B and its transpose RT ⊆ B × A may be composed either as R RT or as RT R. The first composition results in a homogeneous relation on A and the second is on B. Since the images of these hom functors are in Rel itself, in this case hom is an internal hom functor. With its internal hom functor, Rel is a , and furthermore a .
The category Rel can be obtained from the category Set as the for the whose functor corresponds to power set, interpreted as a covariant functor.
Perhaps a bit surprising at first sight is the fact that in Rel is given by the disjoint union (rather than the cartesian product as it is in Set), and so is the coproduct.
Rel is , if one defines both the monoidal product A ⊗ B and the internal hom A ⇒ B by the cartesian product of sets. It is also a if one defines the monoidal product by the disjoint union of sets.
The category Rel was the prototype for the algebraic structure called an by Peter J. Freyd and Andre Scedrov in 1990. Starting with a and a functor F: A → B, they note properties of the induced functor Rel(A,B) → Rel(FA, FB). For instance, it preserves composition, conversion, and intersection.
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In the mathematical field of , an allegory is a that has some of the structure of the category Rel of sets and binary relations between them. Allegories can be used as an abstraction of categories of relations, and in this sense the theory of allegories is a generalization of relation algebra to relations between different sorts. Allegories are also useful in defining and investigating certain constructions in category theory, such as completions. In this article we adopt the convention that morphisms compose from right to left, so RS means "first do S, then do R".
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.
In , a branch of mathematics, compact closed categories are a general context for treating dual objects. The idea of a dual object generalizes the more familiar concept of the dual of a finite-dimensional vector space. So, the motivating example of a compact closed category is FdVect, the having finite-dimensional vector spaces as s and linear maps as s, with tensor product as the structure. Another example is , the category having sets as objects and relations as morphisms, with .
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