Adjoint functor theorems are theorems stating that under certain conditions a functor that preserves limits is a right adjoint, and that a functor that preserves colimits is a left adjoint.
A basic result of category theory is that right adjoint functors preserve all limits that exist in their domain, and, dually, left adjoints preserve all colimits. An adjoint functor theorem is a statement that (under certain conditions) the converse holds: a functor which preserves limits is a right adjoint.
The basic idea of an adjoint functor theorem is that if we could assume that a large category had all limits over small and large diagrams, then for a functor that preserves all these limits we might define its would-be left adjoint by taking to be the limit
This notation stands for a limit over the comma category (whose objects are pairs and whose morphisms are arrows in making the obvious triangle commute in ) of the projection functor that forgets the morphism :
Because with this definition there would be, for every , an obvious morphism
(the component map over of the limiting cone). Moreover, because preserves limits, we would have an isomorphism
(the limit of the functor ), and hence an obvious morphism of cone tips
It is easy to check that these would be the unit and counit of an adjunction . See adjoint functor for more.
The problem with this would-be argument is that in general the comma category may not be small category. But one can generally not expect a large category to have all large limits: even if we pass to a universe in which is considered small, a classical theorem of Freyd says that any complete small category is a preorder (see complete small category for the proof, which is valid in classical logic and also holds classically in any Grothendieck topos). Thus, the argument we gave above is necessarily only an adjoint functor theorem for preorders:
If is any functor between (small) preorders such that has, and preserves, all small meets, then has a left adjoint.
(This theorem holds in constructive mathematics, although not in predicative mathematics; the classical reasoning before this explains why the theorem is not more general, but the proof itself is already constructive.)
To obtain adjoint functor theorems for categories that are not preorders, one must therefore impose various additional “size conditions” on the category and/or the functor .
Sufficient conditions for a limit-preserving functor to be a right adjoint include:
is complete and locally small, and satisfies the solution set condition.
This is Freyd’s original version, sometimes called the “General Adjoint Functor Theorem”.
is complete, locally small, well-powered, and has a small cogenerating set, and is locally small.
This is sometimes called the “Special Adjoint Functor Theorem”, and abbreviated to SAFT.
is locally small and cototal, and is locally small.
In the first two cases, which work by replacing large limits by small ones, it suffices to assume that preserves small limits (that it preserves all limits will follow). The third case works by assuming that has, while not all large limits, enough so that the theorem goes through; thus is this case must be already known to preserve large limits as well.
Here is a proof of the General Adjoint Functor Theorem: that a functor out of a locally small category with all small limits has a left adjoint if it preserves these limits and satisfies the solution set condition.
From the discussion at adjoint functors – In terms of universal arrows we have that the existence of the adjoint is equivalent to the existence for each of an initial object in the comma category : an object such that for each there is a unique such that
commutes. Now an initial object is the limit of the identity functor, but this is generally a large limit; we replace this with some small limit conditions that guarantee existence of an initial object.
Let be a category. Call a small family of objects weakly initial if for every object of there exists and a morphism .
Suppose has small products. If is a weakly initial family, then is a weakly initial object.
Claim: Suppose is locally small and has joint equalizers of small families. If is a weakly initial object, then the domain of the joint equalizer of all arrows is an initial object. Proof: clearly is weakly initial. Suppose given an object and arrows ; we must show . Let be the equalizer of and . There exists an arrow . The arrow equalizes and , so . Since is monic, . Thus is an epi, and follows.
If is locally small and small-complete and preserves limits, then is locally small and small-complete for every object of .
If in addition each has a weakly initial family (solution set condition), then by 2. and 3. each has an initial object. This restates the condition that has a left adjoint.
In fact, it suffices for to be Cauchy complete and for to be -flat for every (i.e. for its Yoneda extension to be continuous). See Borceux.
As before, the proof proceeds by constructing initial objects of comma categories. We assume that is locally small, small-complete, well-powered, has a cogenerating set , and that is a small-continuous functor into a locally small category .
As before, for each object of , the comma category is locally small and small-complete. Moreover, it is easy to check that it is well-powered, and that the set of all objects of the form is a cogenerating set for .
It then remains to prove that any locally small, small-complete, well-powered category with a cogenerating set has an initial object. The initial object is constructed as the intersection = pullback of all subobjects of , i.e., the minimal subobject. Then, given , the equalizer is isomorphic to because is minimal, and so : there is at most one arrow for each .
On the other hand, for each the canonical map
is monic since the cogenerate. The following pullback of ,
gives a subobject of that maps to , and into which embeds. Thus there exists a map , and we conclude is initial.
In practice an important special case is that of functors between locally presentable categories. For these there is the following version of an adjoint functor theorem.
Let be a functor between locally presentable categories. Then
has a right adjoint if and only if it preserves all small colimits.
has a left adjoint if and only if it is an accessible functor and preserves all small limits.
The second statement, characterizing when has a left adjoint, is (AdamekRosicky, theorem 1.66). In the “if” direction, this is an application of the general adjoint functor theorem: any accessible functor satisfies the solution set condition. The “only if”, particularly that having a left adjoint forces accessibility, takes a little work. But in any case there are easy examples that show that continuity alone is insufficient, i.e., examples of continuous functors between locally presentable categories that do not have left adjoints. See below in the section In locally presentable categories.
The first statement, characterizing when has a right adjoint, can be proven using the special adjoint functor theorem: by a non-trivial theorem (AdamekRosicky, theorem 1.58), any locally presentable category is co-wellpowered.
Thus, the first statement can be strengthened by removing the assumption that is locally presentable: it is enough that be locally small. For example, if is locally presentable, then every continuous functor
has a left adjoint (is representable), because its opposite is cocontinuous and therefore has a right adjoint, even though is not locally presentable.
A right adjoint to any cocontinuous functor between locally presentable categories can also be constructed directly. If is locally -presentable and is the subcategory of -small objects, then is equivalent to the full subcategory of of presheaves that preserve -small limits (AdamekRosicky, theorem 1.46). The presheaves in the image of the functor defined by preserve -small limits because is cocontinuous. So this functor factors through the subcategory . The functor so-constructed is a right adjoint to .
The following is a counter-example, indicating the need for something more than just continuity to force a functor between locally presentable categories to be a right adjoint; as stated in theorem , the missing extra condition is precisely accessibility.
For every infinite cardinal number , let be a simple group of cardinality . Define the functor Group Set to be the product of all the representable functors . Since no group can admit a nontrivial homomorphism from proper-class-many of the , this functor does indeed land (or can be redefined to land) in Set. Since it is a product of representables, it is continuous (and of course Group and Set are locally presentable categories), but it is not itself representable (hence has no left adjoint).
André Joyal has been attributing this example to Saunders MacLane; it appears in print for instance right at the beginning of (AdámekKoubekTrnková01).
Suppose and are categories that admit small colimits (i.e., are cocomplete) and is a functor that preserves small colimits (i.e., is cocontinuous). The has a right adjoint if and only if for any object the functor
that sends
is a small presheaf on .
See the MathOverflow answer by Ivan Di Liberti.
Every sheaf topos is a total category and a cototal category.
See the discussion at Grothendieck topos.
It follows that
Let be a functor between sheaf toposes. Then
has a right adjoint precisely if it preserves all small colimits;
has a left adjoint precisely if it preserves all small limits.
It is instructive to spell out the construction of the right adjoint from a colimit preserving functor in the simple case where all categories are categories of presheaves. This is a particularly simple case, but is useful in itself and serves as a template for the general case.
So let now and be small categories and a colimit-preserving functor between their categories of presheaves (which we abbreviate , etc):
Then its right adjoint is given (with denoting the Yoneda embedding) by
as we shall check in a moment. But first notice that using the co-Yoneda lemma this may be rewritten as
where the coend is equivalently given by the colimit
This is the formula for the would-be right adjoint from the general discussion above, only that here the colimit is only over the representables, hence over a small category.
Now we check that the functor thus obtained is indeed right adjoint to , by explicitly checking the hom-isomorphism (here) of the pair of adjoint functors:
We compute . In the first step
we use the co-Yoneda lemma for . Then, because preserves colimits, this is
Since the hom-functor preserves limits in both arguments, we can take the coend out to get an end
Then we use the standard tensoring of our categories over Set to get
And finally this is recognized as the formula for the hom-set of presheaves (see the discussion at functor category):
where on the right we identified (1).
The composition of this sequence of natural isomorphisms is hence the desired hom-isomorphism:
The classical adjoint functor theorems originate in the exercise section of ch.3 (pp.84ff) in
A more recent exposition is in
where the solution set condition is called “pre-adjointness”.
Careful discussions can be found in
Francis Borceux, Handbook of Categorical Algebra I, Cambridge UP, 1994. (sections 3.3, 6.6)
Saunders MacLane, §V.6, V.8 of: Categories for the Working Mathematician, Graduate Texts in Mathematics 5 Springer (1971, second ed. 1997) [doi:10.1007/978-1-4757-4721-8]
A brief introductory discussion is around theorem 5.4 of
A detailed expository survey is
The adjoint functor theorem in context with Yoneda embedding is discussed in
The connection between the solution set condition and the Čech homology construction is discussed in
An enriched adjoint functor theorem is given in:
The parallels between the adjoint functor theorem for categories and the computation of colimits from limits in a lattice as well as similar parallels between co/completeness for Boolean algebras and Paré’s theorem for finite completeness of toposes is studied from a type-theoretical perspective in
The case for locally presentable categories is discussed in
Jiří Adámek, Jiri Rosicky, Locally presentable and accessible categories, Cambridge UP, 1994.
Jiří Adámek, V. Koubek and V. Trnková, How large are left exact functors?, Theory and Applications of Categories 8 (2001), pp. 377-390. (abstract)
A specialization to the locally -presentable case is given in Theorem 2.11 of
A relative version of Freyd’s classical results is in
Adjoint functor theorems for indexed categories are discussed in
A history of the general adjoint functor theorem is discussed in:
A stronger version for finitary functors between locally finitely presentable categories whose domain is ranked, requiring only the preservation of countable limits for the existence of a left adjoint, is discussed in
Last revised on December 2, 2023 at 20:04:51. See the history of this page for a list of all contributions to it.