is defined to be a functor
where denotes the category of presheaves over .
A correspondence from to is a profunctor .
Composition of profunctors
The composite of two profunctors
is given by
It can be shown that
where is the least equivalence relation such that whenever there exists a morphism in such that
- and .
The bicategory of profunctors
Composition of profunctors is associative only up to isomorphism (because the product is not strictly associative in Set). The best one can hope is therefore to build a bicategory Prof whose
Lifting functors to profunctors
A functor can be seen as a profunctor by postcomposing with the Yoneda functor:
It can be shown that such a profunctor has a right adjoint. Moreover, this is a characterization: a profunctor has a right adjoint if and only if factors through the Cauchy completion of , i.e. there exists a functor such that .