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Category theory is a general theory of mathematical structures and their relations that was introduced by Samuel Eilenberg and Saunders Mac Lane in the middle of the 20th century in their foundational work on
algebraic topology Algebraic topology is a branch of mathematics that uses tools from abstract algebra to study topological spaces. The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify ...
. Nowadays, category theory is used in almost all areas of mathematics, and in some areas of
computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
. In particular, many constructions of new mathematical objects from previous ones, that appear similarly in several contexts are conveniently expressed and unified in terms of categories. Examples include quotient spaces,
direct product In mathematics, one can often define a direct product of objects already known, giving a new one. This generalizes the Cartesian product of the underlying sets, together with a suitably defined structure on the product set. More abstractly, one t ...
s, completion, and duality. A category is formed by two sorts of
objects Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ai ...
: the
object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...
s of the category, and the morphisms, which relate two objects called the ''source'' and the ''target'' of the morphism. One often says that a morphism is an ''arrow'' that ''maps'' its source to its target. Morphisms can be ''composed'' if the target of the first morphism equals the source of the second one, and morphism composition has similar properties as function composition ( associativity and existence of identity morphisms). Morphisms are often some sort of function, but this is not always the case. For example, a
monoid In abstract algebra, a branch of mathematics, a monoid is a set equipped with an associative binary operation and an identity element. For example, the nonnegative integers with addition form a monoid, the identity element being 0. Monoid ...
may be viewed as a category with a single object, whose morphisms are the elements of the monoid. The second fundamental concept of category is the concept of a functor, which plays the role of a morphism between two categories C_1 and C_2: it maps objects of C_1 to objects of C_2 and morphisms of C_1 to morphisms of C_2 in such a way that sources are mapped to sources and targets are mapped to targets (or, in the case of a contravariant functor, sources are mapped to targets and ''vice-versa''). A third fundamental concept is a natural transformation that may be viewed as a morphism of functors.


Categories, objects, and morphisms


Categories

A ''category'' ''C'' consists of the following three mathematical entities: * A
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...
ob(''C''), whose elements are called ''objects''; * A class hom(''C''), whose elements are called morphisms or maps or ''arrows''.
Each morphism ''f'' has a ''source object a'' and ''target object b''.
The expression , would be verbally stated as "''f'' is a morphism from ''a'' to ''b''".
The expression – alternatively expressed as , , or – denotes the ''hom-class'' of all morphisms from ''a'' to ''b''. * A binary operation ∘, called ''composition of morphisms'', such that
for any three objects ''a'', ''b'', and ''c'', we have ::. :The composition of and is written as or ''gf'', governed by two axioms: ::1. Associativity: If , , and then ::: ::2.
Identity Identity may refer to: * Identity document * Identity (philosophy) * Identity (social science) * Identity (mathematics) Arts and entertainment Film and television * ''Identity'' (1987 film), an Iranian film * ''Identity'' (2003 film), an ...
: For every object ''x'', there exists a morphism called the '' identity morphism for x'',
such that :::for every morphism , we have :::. :: From the axioms, it can be proved that there is exactly one identity morphism for every object. ::Some authors deviate from the definition just given, by identifying each object with its identity morphism.


Morphisms

Relations among morphisms (such as ) are often depicted using commutative diagrams, with "points" (corners) representing objects and "arrows" representing morphisms. Morphisms can have any of the following properties. A morphism is a: * monomorphism (or ''monic'') if implies for all morphisms . * epimorphism (or ''epic'') if implies for all morphisms . * ''bimorphism'' if ''f'' is both epic and monic. *
isomorphism In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. The word i ...
if there exists a morphism such that . * endomorphism if . end(''a'') denotes the class of endomorphisms of ''a''. * automorphism if ''f'' is both an endomorphism and an isomorphism. aut(''a'') denotes the class of automorphisms of ''a''. * retraction if a right inverse of ''f'' exists, i.e. if there exists a morphism with . * section if a left inverse of ''f'' exists, i.e. if there exists a morphism with . Every retraction is an epimorphism, and every section is a monomorphism. Furthermore, the following three statements are equivalent: * ''f'' is a monomorphism and a retraction; * ''f'' is an epimorphism and a section; * ''f'' is an isomorphism.


Functors

Functors are structure-preserving maps between categories. They can be thought of as morphisms in the category of all (small) categories. A (covariant) functor ''F'' from a category ''C'' to a category ''D'', written , consists of: * for each object ''x'' in ''C'', an object ''F''(''x'') in ''D''; and * for each morphism in ''C'', a morphism in ''D'', such that the following two properties hold: * For every object ''x'' in ''C'', ; * For all morphisms and , . A contravariant functor is like a covariant functor, except that it "turns morphisms around" ("reverses all the arrows"). More specifically, every morphism in ''C'' must be assigned to a morphism in ''D''. In other words, a contravariant functor acts as a covariant functor from the opposite category ''C''op to ''D''.


Natural transformations

A ''natural transformation'' is a relation between two functors. Functors often describe "natural constructions" and natural transformations then describe "natural homomorphisms" between two such constructions. Sometimes two quite different constructions yield "the same" result; this is expressed by a natural isomorphism between the two functors. If ''F'' and ''G'' are (covariant) functors between the categories ''C'' and ''D'', then a natural transformation η from ''F'' to ''G'' associates to every object ''X'' in ''C'' a morphism in ''D'' such that for every morphism in ''C'', we have ; this means that the following diagram is commutative: The two functors ''F'' and ''G'' are called ''naturally isomorphic'' if there exists a natural transformation from ''F'' to ''G'' such that η''X'' is an isomorphism for every object ''X'' in ''C''.


Other concepts


Universal constructions, limits, and colimits

Using the language of category theory, many areas of mathematical study can be categorized. Categories include sets, groups and topologies. Each category is distinguished by properties that all its objects have in common, such as the empty set or the product of two topologies, yet in the definition of a category, objects are considered atomic, i.e., we ''do not know'' whether an object ''A'' is a set, a topology, or any other abstract concept. Hence, the challenge is to define special objects without referring to the internal structure of those objects. To define the empty set without referring to elements, or the product topology without referring to open sets, one can characterize these objects in terms of their relations to other objects, as given by the morphisms of the respective categories. Thus, the task is to find '' universal properties'' that uniquely determine the objects of interest. Numerous important constructions can be described in a purely categorical way if the ''category limit'' can be developed and dualized to yield the notion of a ''colimit''.


Equivalent categories

It is a natural question to ask: under which conditions can two categories be considered ''essentially the same'', in the sense that theorems about one category can readily be transformed into theorems about the other category? The major tool one employs to describe such a situation is called ''equivalence of categories'', which is given by appropriate functors between two categories. Categorical equivalence has found numerous applications in mathematics.


Further concepts and results

The definitions of categories and functors provide only the very basics of categorical algebra; additional important topics are listed below. Although there are strong interrelations between all of these topics, the given order can be considered as a guideline for further reading. * The functor category ''D''''C'' has as objects the functors from ''C'' to ''D'' and as morphisms the natural transformations of such functors. The Yoneda lemma is one of the most famous basic results of category theory; it describes representable functors in functor categories. * Duality: Every statement, theorem, or definition in category theory has a ''dual'' which is essentially obtained by "reversing all the arrows". If one statement is true in a category ''C'' then its dual is true in the dual category ''C''op. This duality, which is transparent at the level of category theory, is often obscured in applications and can lead to surprising relationships. * Adjoint functors: A functor can be left (or right) adjoint to another functor that maps in the opposite direction. Such a pair of adjoint functors typically arises from a construction defined by a universal property; this can be seen as a more abstract and powerful view on universal properties.


Higher-dimensional categories

Many of the above concepts, especially equivalence of categories, adjoint functor pairs, and functor categories, can be situated into the context of ''higher-dimensional categories''. Briefly, if we consider a morphism between two objects as a "process taking us from one object to another", then higher-dimensional categories allow us to profitably generalize this by considering "higher-dimensional processes". For example, a (strict) 2-category is a category together with "morphisms between morphisms", i.e., processes which allow us to transform one morphism into another. We can then "compose" these "bimorphisms" both horizontally and vertically, and we require a 2-dimensional "exchange law" to hold, relating the two composition laws. In this context, the standard example is Cat, the 2-category of all (small) categories, and in this example, bimorphisms of morphisms are simply natural transformations of morphisms in the usual sense. Another basic example is to consider a 2-category with a single object; these are essentially monoidal categories. Bicategories are a weaker notion of 2-dimensional categories in which the composition of morphisms is not strictly associative, but only associative "up to" an isomorphism. This process can be extended for all
natural number In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country"). Numbers used for counting are called '' cardinal ...
s ''n'', and these are called ''n''-categories. There is even a notion of '' ω-category'' corresponding to the ordinal number ω. Higher-dimensional categories are part of the broader mathematical field of higher-dimensional algebra, a concept introduced by Ronald Brown. For a conversational introduction to these ideas, se
John Baez, 'A Tale of ''n''-categories' (1996).


Historical notes

Whilst specific examples of functors and natural transformations had been given by Samuel Eilenberg and Saunders Mac Lane in a 1942 paper on
group theory In abstract algebra, group theory studies the algebraic structures known as groups. The concept of a group is central to abstract algebra: other well-known algebraic structures, such as rings, fields, and vector spaces, can all be seen ...
, these concepts were introduced in a more general sense, together with the additional notion of categories, in a 1945 paper by the same authors (who discussed applications of category theory to the field of
algebraic topology Algebraic topology is a branch of mathematics that uses tools from abstract algebra to study topological spaces. The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify ...
). Their work was an important part of the transition from intuitive and geometric homology to homological algebra, Eilenberg and Mac Lane later writing that their goal was to understand natural transformations, which first required the definition of functors, then categories. Stanislaw Ulam, and some writing on his behalf, have claimed that related ideas were current in the late 1930s in Poland. Eilenberg was Polish, and studied mathematics in Poland in the 1930s. Category theory is also, in some sense, a continuation of the work of Emmy Noether (one of Mac Lane's teachers) in formalizing abstract processes; Noether realized that understanding a type of mathematical structure requires understanding the processes that preserve that structure (
homomorphism In algebra, a homomorphism is a morphism, structure-preserving map (mathematics), map between two algebraic structures of the same type (such as two group (mathematics), groups, two ring (mathematics), rings, or two vector spaces). The word ''homo ...
s). Eilenberg and Mac Lane introduced categories for understanding and formalizing the processes ( functors) that relate topological structures to algebraic structures ( topological invariants) that characterize them. Category theory was originally introduced for the need of homological algebra, and widely extended for the need of modern algebraic geometry ( scheme theory). Category theory may be viewed as an extension of
universal algebra Universal algebra (sometimes called general algebra) is the field of mathematics that studies algebraic structures themselves, not examples ("models") of algebraic structures. For instance, rather than take particular groups as the object of study ...
, as the latter studies algebraic structures, and the former applies to any kind of mathematical structure and studies also the relationships between structures of different nature. For this reason, it is used throughout mathematics. Applications to
mathematical logic Mathematical logic is the study of formal logic within mathematics. Major subareas include model theory, proof theory, set theory, and recursion theory. Research in mathematical logic commonly addresses the mathematical properties of forma ...
and
semantics Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference, meaning, or truth. The term can be used to refer to subfields of several distinct disciplines, including philosophy, linguistics and comput ...
( categorical abstract machine) came later. Certain categories called topoi (singular ''topos'') can even serve as an alternative to axiomatic set theory as a foundation of mathematics. A topos can also be considered as a specific type of category with two additional topos axioms. These foundational applications of category theory have been worked out in fair detail as a basis for, and justification of, constructive mathematics. Topos theory is a form of abstract sheaf theory, with geometric origins, and leads to ideas such as
pointless topology In mathematics, pointless topology, also called point-free topology (or pointfree topology) and locale theory, is an approach to topology that avoids mentioning points, and in which the lattices of open sets are the primitive notions. In this appr ...
. Categorical logic is now a well-defined field based on type theory for intuitionistic logics, with applications in
functional programming In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions tha ...
and domain theory, where a cartesian closed category is taken as a non-syntactic description of a lambda calculus. At the very least, category theoretic language clarifies what exactly these related areas have in common (in some abstract sense). Category theory has been applied in other fields as well. For example,
John Baez John Carlos Baez (; born June 12, 1961) is an American mathematical physicist and a professor of mathematics at the University of California, Riverside (UCR) in Riverside, California. He has worked on spin foams in loop quantum gravity, appli ...
has shown a link between Feynman diagrams in
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
and monoidal categories. Another application of category theory, more specifically: topos theory, has been made in mathematical music theory, see for example the book ''The Topos of Music, Geometric Logic of Concepts, Theory, and Performance'' by Guerino Mazzola. More recent efforts to introduce undergraduates to categories as a foundation for mathematics include those of William Lawvere and Rosebrugh (2003) and Lawvere and Stephen Schanuel (1997) and Mirroslav Yotov (2012).


See also

* Domain theory * Enriched category theory * Glossary of category theory *
Group theory In abstract algebra, group theory studies the algebraic structures known as groups. The concept of a group is central to abstract algebra: other well-known algebraic structures, such as rings, fields, and vector spaces, can all be seen ...
* Higher category theory * Higher-dimensional algebra * Important publications in category theory * Lambda calculus * Outline of category theory * Timeline of category theory and related mathematics


Notes


References


Citations


Sources

* * . * . * * * * * . * * * * * * * * * * * * * * Notes for a course offered as part of the MSc. in
Mathematical Logic Mathematical logic is the study of formal logic within mathematics. Major subareas include model theory, proof theory, set theory, and recursion theory. Research in mathematical logic commonly addresses the mathematical properties of forma ...
, Manchester University. * , draft of a book. * * Based on .


Further reading

*


External links


Theory and Application of Categories
an electronic journal of category theory, full text, free, since 1995.
nLab
a wiki project on mathematics, physics and philosophy with emphasis on the ''n''-categorical point of view.
The n-Category Café
essentially a colloquium on topics in category theory.
Category Theory
a web page of links to lecture notes and freely available books on category theory. * , a formal introduction to category theory. * * , with an extensive bibliography.
List of academic conferences on category theory
* — An informal introduction to higher order categories.
WildCats
is a category theory package for Mathematica. Manipulation and visualization of objects, morphisms, categories, functors, natural transformations, universal properties. * , a channel about category theory. * .
Video archive
of recorded talks relevant to categories, logic and the foundations of physics.
Interactive Web page
which generates examples of categorical constructions in the category of finite sets.

an instruction on category theory as a tool throughout the sciences.
Category Theory for Programmers
A book in blog form explaining category theory for computer programmers.
Introduction to category theory.
{{DEFAULTSORT:Category Theory Higher category theory Foundations of mathematics