# Notes on Categories - 1.1. Deﬁnition and Examples 5 5. The category Ring of rings with unit, such that objRing is the class of all rings with unit and hom Ring(A,B) is the set of

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• Notes on Categories

Rafael Villarroel Flores [email protected]

18th August 2004

• ii

• Contents

I Basic Definitions 1

1 Categories 3 1.1 Definition and Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Small Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3 Constructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4 Subcategories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.5 Special Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.6 Special Morphisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2 Functors 15 2.1 Definition and Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.2 Contravariant Functors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.3 Isomorphism of Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.4 Types of Functors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3 Natural Transformations 23 3.1 Definition and Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.2 The Functor Category . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.3 Equivalence of Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.4 The Yoneda Lemma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

II Limits 31

4 Limits and Colimits 33 4.1 Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4.2 Colimits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 4.3 More Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 4.4 Limit and Colimit as Functors . . . . . . . . . . . . . . . . . . . . . . . . . 39 4.5 Preservation of Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

iii

• iv Contents

5 Universals and Adjoints 43 5.1 Universals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 5.2 Adjoint Functors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

6 More on Limits 49 6.1 Limits in a Functor Category . . . . . . . . . . . . . . . . . . . . . . . . . . 49 6.2 Ends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 6.3 Ends in a Functor Category . . . . . . . . . . . . . . . . . . . . . . . . . . 56 6.4 Iterated Ends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 6.5 Coends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

III Extras 65

7 Abelian Categories 67 7.1 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 7.2 Chain Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 7.3 Exact Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 7.4 Homology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 7.5 Split Chain Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 7.6 Exact Functors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

8 Appendix: Calculations 75 8.1 The Grothendieck Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Bibliography 77

• Part I

Basic Definitions

1

• 1 Categories

1.1 Definition and Examples

In this section we provide the definition of category and many examples of categories which may already be familiar to the reader.

1.1 Definition. A category C consists of

1. A class obj C, whose elements are called objects.

2. A set homC(A,B) for every pair of objects A,B, whose elements are called mor- phisms, or maps from A to B. We can call them also C-maps.

3. For every triad of objects A,B,C, a function (called composition)

homC(A,B)× homC(B,C) → homC(A,C) (1.1)

whose value at (f,g) will be denoted by g ◦ f.

4. For every object A, a distinguished element 1CA ∈ homC(A,A), called identity on A.

They have to satisfy the following conditions:

(a) For every pair of objectsA,B and for every f ∈ homC(A,B), we must have f◦1CA = 1CB ◦ f = f.

(b) For every A,B,C,D ∈ obj D and f ∈ homC(A,B), g ∈ homC(B,C) and h ∈ homC(C,D) we have that h ◦ (g ◦ f) = (h ◦ g) ◦ f.

3

• 4 Chapter 1. Categories

An identity 1CA is usually denoted by 1A if the category C is clear from the context. If A and B are objects in the category C, then f : A → B will mean that f is an element of homC(A,B). In that case, we say that A is the domain of f and that B is the codomain of f, and we write A = dom f, B = cod f.

An element of obj C will be called a C-object, similarly a morphism in C will be called a C-morphism.

We note that we always consider different hom sets to be disjoint. That is, if f,g are morphisms in a category C such that f = g, with f ∈ homC(A,B), g ∈ homC(C,D), then we must have that A = C, B = D.

Our first example is the prototype of all categories.

Example 1.1 The category Set of sets, such that obj Set is the class of all sets and homSet(A,B) is the set of all maps from A to B. Composition is the usual composition of maps, and for a set X, the identity 1X is the identity map. Then clearly the conditions (a) and (b) of the definition of categories are satisfied.

In the following examples of categories, the objects are sets with some extra struc- ture, and morphisms are maps which preserve that structure. Those categories are called concrete, in them, the compositions are given by the usual composition of maps, and the identities are the identity maps. Since the usual composition is associative and the identity map is a neutral element under composition, in order to prove that we have a category one only has to check that the composition of two morphisms is a morphism, and that the identity map is a morphism.

Examples from Algebra

Examples 1.2 1. The category Grp of all groups, where we have that obj Grp is the class of all groups and

homGrp(A,B) is the set of group homomorphisms from A to B.

2. Fixing a group G, we have the category GSet, where obj GSet is the class of (left) G-sets and homGSet(X, Y) is the set of equivariant maps.

3. The category Ab of all abelian groups, such that obj Ab is the class of all abelian groups and homAb(A,B) is the set of group homomorphisms from A to B.

4. The category Rng of rings, such that obj Rng is the class of all rings, homRng(A,B) is the set of ring homomorphisms from A to B.

• 1.1. Definition and Examples 5

5. The category Ring of rings with unit, such that obj Ring is the class of all rings with unit and homRing(A,B) is the set of all ring homomorphisms from A to B such that f(1) = 1.

6. The category R-mod, where R is a ring, such that objR-mod is the class of all left modules over R and homR-mod(A,B) is the set of R-linear morphisms from A to B. We similarly have a category mod-R of right R-modules. Note that if R is in fact a field, then R-mod is the category of vector spaces over R, and the morphisms in this particular case are linear transformations.

7. The category R-alg of R-algebras, such that objR-alg is the class of all algebras over R and homR-alg(A,B) is the set of morphisms of R-algebras from A to B.

Examples from Topology

Examples 1.3 1. The category Top of topological spaces, such that obj Top is the class of all topological

spaces and homTop(X, Y) is the set of continuous maps from X to Y.

2. The category Top∗ of pointed topological spaces, such that obj Top∗ is the class of all pairs (X, x0), where X is a topological space and x0 is a point in X. We put homTop∗

( (X, x0), (Y,y0)

) as the set of continuous maps from X to Y such that f(x0) = y0.

3. The category Haus of Hausdorff topological spaces, such that obj Haus is the class of all Hausdorff topological spaces and homHaus(X, Y) is the set of all continuous maps from X to Y.

4. The category Metric of metric spaces, such that obj Metric is the class of all metric spaces and homMetric(X, Y) is the set of all continuous maps (satisfying an ε-δ definition) from X to Y.

Examples from Combinatorics

Examples 1.4 1. The category Poset, with class of objects the class of partially ordered sets and

homPoset(P,Q) is the set of monotone maps from P to Q.

2. The category Graph where obj Graph is the class of all graphs, and for G1,G2 ∈ obj Graph, we have that homGraph(G1,G2) is the set of functions from G1 to G2 that preserve adjacency.

3. The category DirGraph where obj DirGraph is the class of all directed graphs, and for G1,G2 ∈ obj Graph, we have that homGraph(G1,G2) is the set of functions f such that whenever a→ b is and edge in G1, then f(a) → f(b) is an edge in G2.

4. The category SimplComplex, with class of objects the class of abstract simplicial complexes and homSimplComplex(K,L) is the set of simplicial maps from K to L.

• 6 Chapter 1. Categories

At this point, we should now provide examples of categories which are not concrete. In the following examples, either the objects are not sets with structure or the hom sets are not composed by maps between sets.

Examples 1.5 1. We define a category mod where obj mod consists of pairs (R,M) where R is a ring and M

is a left R-module. A morphism (R,M) → (S,N) in mod is a pair of maps (φ, f), where φ : R → S is a morphism of rings and f : M → N is an additive map

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