Category (mathematics)

In mathematics, categories allow one to formalize notions involving abstract structure and processes which preserve structure. Categories appear in virtually every branch of modern mathematics and are a central unifying notion. The study of categories in their own right is known as category theory.
For more extensive motivational background and historical notes, see category theory and the list of category theory topics.
Contents 
Definition
A category C consists of
 a class ob(C) of objects:
 a class hom(C) of morphisms. Each morphism f has a unique source object a and target object b. We write f: a → b, and we say "f is a morphism from a to b". We write hom(a, b) (or hom_{C}(a, b)) to denote the homclass of all morphisms from a to b. (Some authors write Mor(a, b).)
 for every three objects a, b and c, a binary operation hom(a, b) × hom(b, c) → hom(a, c) called composition of morphisms; the composition of f : a → b and g : b → c is written as g o f or gf (Some authors write fg.)
such that the following axioms hold:
 (associativity) if f : a → b, g : b → c and h : c → d then h o (g o f) = (h o g) o f, and
 (identity) for every object x, there exists a morphism 1_{x} : x → x called the identity morphism for x, such that for every morphism f : a → b, we have 1_{b} o f = f = f o 1_{a}.
From these axioms, one can prove that there is exactly one identity morphism for every object. Some authors use a slight variation of the definition in which each object is identified with the corresponding identity morphism.
A small category is a category in which both ob(C) and hom(C) are actually sets and not proper classes. A category which is not small is said to be large. A locally small category is a category such that for all objects a and b, the homclass hom(a, b) is a set. Many important categories in mathematics (such as the category of sets), although not small, are at least locally small.
The morphisms of a category are sometimes called arrows due to the influence of commutative diagrams.
Examples
Each category is presented in terms of its objects, its morphisms, and its composition of morphisms.
 The category Set of all sets together with functions between sets, where composition is the usual function composition (The following are subcategories of Set, obtained by adding some type of structure onto a set, by requiring that morphisms are functions which respect this added structure, and where morphism composition is simply ordinary function composition.)
 The category Ord of all preordered sets with monotonic functions
 The category Mag consisting of all magmas with their homomorphisms
 The category Med consisting of all medial magmas with their homomorphisms
 The category Grp consisting of all groups with their group homomorphisms
 The category Ab consisting of all abelian groups with their group homomorphisms
 The category Vect_{K} of all vector spaces over the field K (which is held fixed) with their Klinear maps
 The category Top of all topological spaces with continuous functions
 The category Met of all metric spaces with short maps
 The category Uni of all uniform spaces with uniformly continuous functions
 The category Cat of all small categories with functors
 Any preordered set (P, ≤) forms a small category, where the objects are the members of P, the morphisms are arrows pointing from x to y when x ≤ y (The composition law is forced, because there is at most one morphism from any object to another.)
 Any monoid forms a small category with a single object x. (Here, x is any fixed set.) The morphisms from x to x are precisely the elements of the monoid, and the categorical composition of morphisms is given by the monoid operation. In fact, one may view categories as generalizations of monoids; several definitions and theorems about monoids may be generalized for categories.
 Any directed graph generates a small category: the objects are the vertices of the graph and the morphisms are the paths in the graph. Composition of morphisms is concatenation of paths. This is called the free category generated by the graph.
 If I is a set, the discrete category on I is the small category which has the elements of I as objects and only the identity morphisms as morphisms. Again, the composition law is forced.)
 Any category C can itself be considered as a new category in a different way: the objects are the same as those in the original category but the arrows are those of the original category reversed. This is called the dual or opposite category and is denoted C^{op}.
 If C and D are categories, one can form the product category C × D: the objects are pairs consisting of one object from C and one from D, and the morphisms are also pairs, consisting of one morphism in C and one in D. Such pairs can be composed componentwise.
Types of morphisms
A morphism f : a → b is called
 a monomorphism (or monic) if fg_{1} = fg_{2} implies g_{1} = g_{2} for all morphisms g_{1}, g_{2} : x → a.
 an epimorphism (or epic) if g_{1}f = g_{2}f implies g_{1} = g_{2} for all morphisms g_{1}, g_{2} : b → x.
 an isomorphism if it is both monic and epic; equivalently, if there exists a morphism g : b → a with fg = 1_{b} and gf = 1_{a}.
 an endomorphism if a = b. The class of endomorphisms of a is denoted end(a).
 an automorphism if f is both an endomorphism and an isomorphism. The class of automorphisms of a is denoted aut(a).
Relations among morphisms (such as fg = h) can most conveniently be represented with commutative diagrams, where the objects are represented as points and the morphisms as arrows.
Types of categories
 In many categories, the homsets hom(a, b) are not just sets but actually abelian groups, and the composition of morphisms is compatible with these group structures; i.e. is bilinear. Such a category is called preadditive. If, furthermore, the category has all finite products and coproducts, it is called an additive category. If all morphisms have a kernel and a cokernel, and all epimorphism are cokernels and all monomorphisms are kernels, then we speak of an abelian category. A typical example of an abelian category is the category of abelian groups.
 A category is called complete if all limits in it exist. The categories of sets, abelian groups and topological spaces are complete.
 A category is called cartesian closed if it has finite direct products and a morphism defined on a finite product can always be represented by a morphism defined on just one of the factors.
 A topos is a certain type of cartesian closed category in which all of mathematics can be formulated (just like classically all of mathematics is formulated in the category of sets). A topos can also be used to represent a logical theory.
 A groupoid is a category in which every morphism is an isomorphism. Groupoids are generalizations of groups, group actions and equivalence relations.
References
 Adámek, Jiří, Herrlich, Horst, & Strecker, George E. (1990). Abstract and Concrete Categories (http://katmat.math.unibremen.de/acc/acc.pdf). Originally publ. John Wiley & Sons. ISBN 0471609226. (now free online edition)
 Asperti, Andrea, & Longo, Giuseppe (1991). Categories, Types and Structures (ftp://ftp.di.ens.fr/pub/users/longo/CategTypesStructures/book.pdf). Originally publ. M.I.T. Press.
 Barr, Michael, & Wells, Charles (2002). Toposes, Triples and Theories (http://www.cwru.edu/artsci/math/wells/pub/ttt.html). (revised and corrected free online version of Grundlehren der mathematischen Wissenschaften (278). SpringerVerlag,1983)
 Borceux, Francis (1994). Handbook of Categorical Algebra.. Vols. 5052 of Encyclopedia of Mathematics and its Applications. Cambridge: Cambridge University Press.
 Lawvere, William, & Schanuel, Steve. (1997). Conceptual Mathematics: A First Introduction to Categories. Cambridge: Cambridge University Press.
 Mac Lane, Saunders (1998). Categories for the Working Mathematician (2nd ed.). Graduate Texts in Mathematics 5. Springer. ISBN 0387984038.
External links
 "Category Theory" in Stanford Encyclopedia of Philosophy (http://plato.stanford.edu/entries/categorytheory/)
 Homepage of the Categories mailing list (http://www.mta.ca/~catdist/categories.html), with extensive list of resources
 Category Theory section of Alexandre Stefanov's list of free online mathematics resources (http://us.geocities.com/alex_stef/mylist.html)de:Kategorientheorie
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