In Category Theory we introduced the language of categories, and in many posts in this blog we have seen how useful it is in describing concepts in modern mathematics, for example in the two most recent posts, The Theory of Motives and Algebraic Spaces and Stacks. In this post, we introduce another important concept in category theory, that of adjoint functors, as well as the closely related notion of monads. Manifestations of these ideas are quite ubiquitous in modern mathematics, and we enumerate a few examples in this post.
An adjunction between two categories and is a pair of functors, , and , such that there exists a bijection
for all objects of and all objects of . We say that is left-adjoint to , and that is right-adjoint to . We may also write .
An adjunction determines two natural transformations and , called the unit and counit, respectively. Conversely, the functors and , together with the natural transformations and , are enough to determine the adjunction, therefore we can also denote the adjunction by .
We give an example of an adjunction. Let be a fixed field, and consider the functors
where is the functor which assigns to a set the vector space made up of formal linear combinations of elements of with coefficients in ; in other words, an element of can be written as , where and , and is the forgetful functor, which assigns to a vector space the set of elements (vectors) of ; in other words it simply “forgets” the vector space structure on .
For every function in we have a linear transformation in given by . The correspondence has an inverse , given by restricting to (so that our only linear transformations are of the form , and we can obtain set-theoretic functions corresponding to these linear transformations). Hence we have a bijection
We therefore see that the two functors and form an adjunction; the functor (sometimes called the free functor) is left-adjoint to the forgetful functor , and is right-adjoint to .
As another example, consider now the category of modules over a commutative ring , and the functors and (see The Hom and Tensor Functors). For every morphism we have another morphism given by . We actually have a bijection
This is called the Tensor-Hom adjunction.
Closely related to the concept of an adjunction is the concept of a monad. A monad is a triple where is a functor from to itself, is a natural transformation from to , and is a natural transformation from , satisfying the following properties:
Dual to the concept of a monad is the concept of a comonad. A comonad on a category may be thought of as a monad on the opposite category .
As an example of a monad, we can consider the action of a fixed group on a set (such as the symmetric group permuting the elements of the set, for example). In this case, our category will be , and , , and are given by
Adjunctions and monads are related in the following way. Let and be a pair of adjoint functors with unit and counit . Then we have a monad on given by . We can also obtain a comonad given by .
Conversely, if we have a monad on the category , we can obtain a pair of adjoint functors and , where is the Eilenberg-Moore category, whose objects (called -algebras) are pairs , where is an object of , and is a morphism satisfying
and whose morphisms are morphisms in such that
In the example we gave above in the discussion on monads, the -algebras are exactly the sets with the action of the group . If is such a set, then the corresponding -algebra is the pair , where the function satisfies
For comonads, we have a dual notion of coalgebras. These “dual” ideas are important objects of study in themselves, for example in topos theory. Another reason to consider comonads and coalgebras is that in mathematics there often arises a situation where we have three functors
where is left-adjoint to , and is right-adjoint to (a so-called adjoint triple). As an example, consider the forgetful functor which assigns to a topological space its underlying set. It has both a left-adjoint which assigns to a set the trivial topology (where the only open sets are the empty set and itself), and a right-adjoint which assigns to the set the discrete topology (where every subset of is an open set). Therefore we have a monad and a comonad on given by and respectively.
Many more examples of adjoint functors and monads can be found in pretty much all areas of mathematics. And according to a principle attributed to the mathematician Saunders Mac Lane (one of the founders of category theory, along with Samuel Eilenberg), such a structure that occurs widely enough in mathematics deserves to be studied for its own sake.
Categories for the Working Mathematician by Saunders Mac Lane
Category Theory by Steve Awodey