# NOTES ON CATEGORIES AND FUNCTORS

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1 NOTES ON CATEGORIES AND FUNCTORS These notes collect basic definitions and facts about categories and functors that have been mentioned in the Homological Algebra course. For further reading about category theory, consult [4]. Definition 1. A category C consists of 1. Categories and functors A collection Ob C of objects. For every two objects X and Y in C a set of morphisms Hom C (X, Y ). For any triple of objects X, Y, Z a composition function subject to the following conditions: : Hom C (Y, Z) Hom C (X, Y ) Hom C (X, Z) g, f g f, (C1) The composition is associative: Given morphisms X f Y g Z h W we have that h (g f) = (h g) f. (C2) For every object X in C there is an identity morphism 1 X : X X with the property that 1 X f = f, g 1 X = g for any morphisms f : Y X and g : X Z. (C3) The sets Hom C (X, Y ) and Hom C (X, Y ) are disjoint unless X = X and Y = Y. Remark 2. Condition (C3) simply means that the domain and codomain of a morphism is determined by the morphism. If e 1 and e 2 are morphisms from X to itself that satisfy the conditions for an identity morphism, i.e., e i f = f and g e i = g for any morphisms f and g, then e 1 = e 1 e 2 = e 2, so we may speak of the identity morphism 1 X of the object X. For objects X and Y in a category C we will write, and we have already written, f : X Y or X f Y to indicate that f is an element of the set Hom C (X, Y ). Example 3. S is the category of sets, whose objects are sets and whose morphisms are functions between sets. R Mod is the category of left modules over an associative unital ring R, whose objects are all left R-modules and whose morphisms are homomorphisms of left R-modules. Similarly, Mod R denotes the category of right modules over R. Top is the category of topological spaces, whose objects are all topological spaces and whose morphisms are continuous maps. A category C with only one object is the same thing as a monoid M = Hom C (, ) with the composition of C as multiplication. A category could therefore be thought of as a monoid with several objects. 1

2 2 NOTES ON CATEGORIES AND FUNCTORS A category C is called small if the collection of objects Ob C forms a set. The category of sets is not a small category (Russel s paradox), but the category of subsets of a given set would be an example of a small category. Definition 4. Let C and D be categories. A (covariant) functor F : C D consists of For every object X in C an object F (X) in D. For every morphism f : X Y in C a morphism F (f): F (X) F (Y ) in D. subject to the following conditions: g (F1) Given morphisms X f Y Z, we have that F (g) F (f) = F (g f). (F2) For any object X in C, we have that F (1 X ) = 1 F (X). Definition 5. Let F, G: C D be functors. A natural transformation η : F G consists of For every object X in C a morphism η X : F (X) G(X) in D. subject to the following condition: (N1) For every morphism f : X Y in C the diagram is commutative. F (X) F (f) F (Y ) η X G(X) G(f) η Y G(Y ) That η is a natural transformation from F to G may be pictorially indicated as: C F η G D 2. Special morphisms A morphism f : X Y is called an isomorphism if there is a morphism g : Y X such that f g = 1 Y and g f = 1 X. A morphism f in a category C is called a monomorphism if f g = f h implies g = h. It is called an epimorphism if g f = h f implies g = h. Any isomomorphism is necessarily both a monomorphism and an epimorphism, but the converse need not be true. A category is called balanced if any morphism which is both a monomorphism and an epimorphism is an isomorphism. 3. Constructions on categories Opposite category. The opposite category, or dual category, of C is the category C op whose objects are the same as those of C but where morphisms are reversed in the sense that Hom C op(x, Y ) = Hom C (Y, X) for any objects X and Y. The composition op : Hom C op(y, Z) Hom C op(x, Y ) Hom C op(x, Z) is defined using the composition in C: Given composable morphisms X f Y g Z

3 NOTES ON CATEGORIES AND FUNCTORS 3 in C op, this is by definition the same thing as morphisms in C, and we define X f Y g g op f = f g. This is a morphism from Z to X in C, in other words a morphism from X to Z in C op. Definition 6. A contravariant functor F : C D is a functor F : C op D. A contravariant functor from C to D can also be thought of as a functor from C to D op. Product category. Let C and D be categories. The product category C D has objects Ob(C D) = Ob C Ob D and morphisms Hom C D ((X, X ), (Y, Y )) = Hom C (X, Y ) Hom D (X, Y ). Composition is defined componentwise: Given morphisms (X, X ) (f,f ) (Y, Y ) (g,g ) (Z, Z ) we set (g, g ) (f, f ) = (g f, g f ). The category of categories. Ignoring set theoretical issues, we can define the category Cat whose objects are all categories, and where morphisms are given by Hom Cat (C, D) = {functors F : C D}. The composition of functors is defined by (G F )(X) = G(F (X)) on objects X of C and by (G F )(f) = G(F (f)) on morphisms f in C. The identity functor 1 C : C C is given by 1 C (X) = X and 1 C (f) = f for all objects X and all morphisms f in C. The category of functors between two categories. Let C and D be categories. Given natural transformations F η Z G θ H between functors F, G, H : C D, we can define the composite natural transformation θ η : F H by (θ η) X = θ X η X for objects X in C. For every functor F there is an identity natural transformation 1 F : F F defined by (1 F ) X = 1 F (X) for all objects X in C. That the composition of natural transformations is associative follows from the associativity of the composition in D. The functor category D C has objects all functors F : C D and morphisms Hom D C(F, G) = {natural transformations η : F G}. 4. Equivalences of categories and adjoint functors Definition 7. Let F, G: C D be functors. A natural transformation η : F G is called a natural isomorphism, or an isomorphism of functors, if η X : F (X) G(X) is an isomorphism in D for every object X in C. If F and G are functors such that there exists a natural isomorphism η : F G, then we say that F and G are isomorphic, and we write F = G.

4 4 NOTES ON CATEGORIES AND FUNCTORS Definition 8. A functor F : C D is called an equivalence of categories if there is a functor G: D C such that there are isomorphisms of functors F G = 1 D, G F = 1 C. In this situation, F is called an isomorphism of categories if G can be chosen so that F G = 1 D and G F = 1 C. Definition 9. Let C and D be categories. An adjunction consists of two functors C F D such that there is a natural isomorphism of functors from C op D to S G η X,Y : Hom D (F (X), Y ) = Hom C (X, G(Y )). In this situation, we say that F is left adjoint to G and that G is right adjoint to F. Sometimes, this is written F G. Example 10. Let R and S be rings, and let M be an R-S-bimodule. Then there are two important adjunctions: (1) If L is a right R-module then L R M is a right S-module via (l m)s = l (ms), and if N is a right S-module then Hom S op(m, N) is a right R- module via (φr)(m) = φ(rm). There is an adjunction Mod R R M Mod S Hom S op (M, ) η L,N : Hom S op(l R M, N) = Hom R op(l, Hom S op(m, N)) η L,N (φ)(l)(m) = φ(l m). (2) If L is a left R-module then Hom R (L, M) is a right S-module via (φs)(l) = φ(l)s, and if N is a right S-module then Hom S op(n, M) is a left R-module via (rφ)(n) = rφ(n). There is an adjunction R Mod Hom R (,M) (Mod S ) op Hom S op (,M) θ L,N : Hom S op(n, Hom R (L, M)) θ L,N (φ)(l)(n) = φ(n)(l). = Hom R (L, Hom S op(n, M)) Definition 11. A functor F : C D is called faithful, or full respectively, if the function Hom C (X, Y ) Hom D (F (X), F (Y )) f F (f) is injective, or surjective respectively. Sometimes, a functor which is both full and faithful is called fully faithful.

5 NOTES ON CATEGORIES AND FUNCTORS 5 Definition 12. An object X in a category C is called a generator if the functor Hom C (X, ): C S is faithful. Dually, X is called a cogenerator if the functor is faithful. Hom C (, X): C op S 5. Constructions within categories Constructions within a category are often defined by universal properties. A construction is characterized by how it maps out of, or into, other objects in the category. Pinning down a construction by means of a universal property ensures that the construction is determined in the strongest possibly way, namely up to unique isomorphism. However, existence of a certain construction must be proved separately in each case. This is usually done by writing down an explicit formula. Definition 13. A direct product of a family of objects {X i } i I is an object X together with morphisms π i : X X i, called projections, that satisfy the following universal property: Given an object Y and morphisms φ i : Y X i for every i I, there is a unique morphism ψ : Y X such that the diagram commutes for every i I. Y ψ X π i φ i As usual when something is defined by a universal property, it is unique up to unique isomorphism. Therefore, one can allow oneself to use the notation X = i I X i if X is a product of the family of objects {X i }. Note however that the direct product is more than the object i I X i the projections π i : i I X i X i are part of the data. Definition 14. A coproduct, or direct sum, of a family of objects {X i } i I is an object X together with morphisms ι i : X i X, called injections, that satisfy the following universal property: Given an object Y and morphisms µ i : X i Y, there is a unique morphism θ : X Y such that the diagram commutes for every i I. X i X i X ι i θ µ i Y If X is a direct sum of the family {X i }, then we use the coproduct notation X = i I X i or the direct sum notation X = i I X i. The direct sum notation is commonly used in additive categories, and the coproduct notation is used in non-additive categories. Definition 15. A terminal object is an object that satisfies the following universal property: For every object Y there is a unique morphism t Y : Y. An initial object is an object that satisfies the following universal property: For every object Y there is a unique morphism i Y : Y.

6 6 NOTES ON CATEGORIES AND FUNCTORS A zero object is an object 0 which is both a terminal and an initial object. Note that a terminal object is a product and an initial object a coproduct of the empty family of objects. A category is called pointed if it possesses a zero object. A category is pointed if and only if it has a terminal and an initial object and the unique morphism is an isomorphism; in this case both and are zero objects. For any two objects X and Y in a pointed category, the zero morphism 0: X Y is defined to be the composite X t X i Y 0 Y. Definition 16. Let f : X Y be a morphism in a pointed category C. A kernel of f is an object K together with a morphism κ: K X such that f κ = 0 and which satisfy the following universal property: Given an object Z and a morphism ζ : Z X such that f ζ = 0, there is a unique morphism λ: Z K such that the diagram commutes. Z λ ζ K κ X f Y Definition 17. Let f : X Y be a morphism in a pointed category C. A cokernel of f is an object C together with a morphism γ : Y C such that γ f = 0 which satisy the following universal property: Given an object Z and a morphism ζ : Y Z such that ζ f = 0, there is a unique morphism λ: C Z such that the diagram commutes. X f Y C ζ λ Z γ 6. Additive and abelian categories Definition 18. A pre-additive category is a category A where the set of morphisms Hom A (X, Y ) between any two objects X and Y has the structure of an abelian group, and moreover the composition Hom A (Y, Z) Hom A (X, Y ) Hom A (X, Z) is bilinear, i.e., (g + g ) f = g f + g f and g (f + f ) = g f + g f for any morphisms f, f : X Y and g, g : Y Z. A pre-addivite category A with one object is the same thing as an associative unital ring R = Hom A (, ). The ring multiplication is the composition in A. Therefore, a pre-additive category could be thought of as a ring with many objects. The opposite category A op corresponds to the opposite ring R op. Definition 19. An additive functor between pre-additive categories A and B is a functor F : A B such that for every two objects X and Y in A, the function Hom A (X, Y ) Hom B (F (X), F (Y )) is a homomorphism of abelian groups, i.e., F (f + g) = F (f) + F (g) for any morphisms f, g : X Y.

8 8 NOTES ON CATEGORIES AND FUNCTORS If A is an abelian category, then so is the opposite category A op. In particular, the opposite category ( R Mod) op of left modules over a ring R is abelian. This is a simple example of an abelian category which is not equivalent to a module category. If A is abelian and if I is any small category, then the functor category A I is abelian. Sums, products, kernels and cokernels, etc, are defined pointwise. For any abelian category A one can define the category Com(A) of complexes of objects in A. The category Com(A) is again abelian, by defining sums, products, etc, levelwise. The category Sh(X) of sheaves on a topological space X is abelian. It has enough injectives, but it does not have enough projectives. Abelian categories are the proper setting in which to do homological algebra. However, in proving diagram lemmas like the Five Lemma, or when deriving the long exact homology sequence from a short exact sequence of complexes, one selects elements from the modules involved and then one chases them around the diagram at hand. The objects in an abstract abelian category are not necessarily sets with extra structure, so it does not make sense to talk about elements. Since there are examples of abelian categories that are not equivalent to module categories, this is a real problem. It is possible to develop techniques for performing diagram chases in arbitrary abelian categories (ask AB he will be happy to tell you about it), and these techniques suffice to develop the elements of homological algebra. There is also another, more sophisticated, approach. Even though not every abelian category is equivalent to a module category, one can find an exact embedding of every small abelian category into a module category. Theorem 24 (Freyd-Mitchell Embedding Theorem). If A is a small abelian category, then there is a ring R and a fully faithful exact functor A R Mod. In particular, when proving a diagram lemma, one can restrict attention to a suitable small abelian subcategory. A consequence of the embedding theorem is the following metatheorem. Theorem 25 (Metatheorem). Any diagram lemma that is true in all categories of R-modules is true in all abelian categories. These theorems are proved in the book [1]. For instance, the metatheorem applies to the Five Lemma, the (3 3)-Lemma, the Snake Lemma, the Horseshoe Lemma, etc. This situation is in a sense somewhat similar to the theory of finite dimensional vector spaces over a field k. In a course on linear algebra, you first deal with k n and linear maps, aka matrices, between these. Then you develop an axiomatic theory of abstract vector spaces, and you encounter some exotic examples that do not look very much like k n. But then you learn that every finite dimensional vector space is actually isomorphic to k n for some n, so in proving statements about finite dimensional vector spaces, you might as well assume that they are of this form. Similarly, there are definitely quite exotic examples of abelian categories that bear little or no resemblance to the category of modules over a ring. However, the Freyd-Mitchell Embedding Theorem tells you that when working within an abelian category, you might as well assume that you are in a module category. References [1] P. Freyd, Abelian Categories, Harper & Row, [2] P. Gabriel, Des catégories abéliennes, Bull. Soc. Math. France 90 (1962) [3] A. Grothendieck, Sur quelques points d algèbre homologique, Tohoku Math. J. (2) 9 (1957)

9 NOTES ON CATEGORIES AND FUNCTORS 9 [4] S. MacLane, Categories for the Working Mathematician, Graduate texts in Mathematics 5, Springer, 1971.

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