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**Category theory** formalizes mathematical structure and its concepts in terms of a labeled directed graph called a * category *, whose nodes are called *objects*, and whose labelled directed edges are called *arrows* (or morphisms).^{ [1] } A category has two basic properties: the ability to compose the arrows associatively, and the existence of an identity arrow for each object. The language of category theory has been used to formalize concepts of other high-level abstractions such as sets, rings, and groups. Informally, category theory is a general theory of functions.

- Basic concepts
- Applications of categories
- Utility
- Categories, objects, and morphisms
- Functors
- Natural transformations
- Categories, objects, and morphisms 2
- Categories
- Morphisms
- Functors 2
- Natural transformations 2
- Other concepts
- Universal constructions, limits, and colimits
- Equivalent categories
- Further concepts and results
- Higher-dimensional categories
- Historical notes
- See also
- Notes
- References
- Citations
- Sources
- Further reading
- External links

Several terms used in category theory, including the term "morphism", are used differently from their uses in the rest of mathematics. In category theory, morphisms obey conditions specific to category theory itself.

Samuel Eilenberg and Saunders Mac Lane introduced the concepts of categories, functors, and natural transformations from 1942–45 in their study of algebraic topology, with the goal of understanding the processes that preserve mathematical structure.

Category theory has practical applications in programming language theory, for example the usage of monads in functional programming. It may also be used as an axiomatic foundation for mathematics, as an alternative to set theory and other proposed foundations.

Categories represent abstractions of other mathematical concepts. Many areas of mathematics can be formalised by category theory as categories. Hence category theory uses abstraction to make it possible to state and prove many intricate and subtle mathematical results in these fields in a much simpler way.^{ [2] }

A basic example of a category is the category of sets, where the objects are sets and the arrows are functions from one set to another. However, the objects of a category need not be sets, and the arrows need not be functions. Any way of formalising a mathematical concept such that it meets the basic conditions on the behaviour of objects and arrows is a valid category—and all the results of category theory apply to it.

The "arrows" of category theory are often said to represent a process connecting two objects, or in many cases a "structure-preserving" transformation connecting two objects. There are, however, many applications where much more abstract concepts are represented by objects and morphisms. The most important property of the arrows is that they can be "composed", in other words, arranged in a sequence to form a new arrow.

Categories now appear in many branches of mathematics, some areas of theoretical computer science where they can correspond to types or to database schemas, and mathematical physics where they can be used to describe vector spaces.^{ [3] } Probably the first application of category theory outside pure mathematics was the "metabolism-repair" model of autonomous living organisms by Robert Rosen.^{ [4] }

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The study of categories is an attempt to *axiomatically* capture what is commonly found in various classes of related mathematical structures by relating them to the *structure-preserving functions* between them. A systematic study of category theory then allows us to prove general results about any of these types of mathematical structures from the axioms of a category.

Consider the following example. The class **Grp** of groups consists of all objects having a "group structure". One can proceed to prove theorems about groups by making logical deductions from the set of axioms defining groups. For example, it is immediately proven from the axioms that the identity element of a group is unique.

Instead of focusing merely on the individual objects (e.g., groups) possessing a given structure, category theory emphasizes the morphisms – the structure-preserving mappings – *between* these objects; by studying these morphisms, one is able to learn more about the structure of the objects. In the case of groups, the morphisms are the group homomorphisms. A group homomorphism between two groups "preserves the group structure" in a precise sense; informally it is a "process" taking one group to another, in a way that carries along information about the structure of the first group into the second group. The study of group homomorphisms then provides a tool for studying general properties of groups and consequences of the group axioms.

A similar type of investigation occurs in many mathematical theories, such as the study of continuous maps (morphisms) between topological spaces in topology (the associated category is called **Top**), and the study of smooth functions (morphisms) in manifold theory.

Not all categories arise as "structure preserving (set) functions", however; the standard example is the category of homotopies between pointed topological spaces.

If one axiomatizes relations instead of functions, one obtains the theory of allegories.

A category is *itself* a type of mathematical structure, so we can look for "processes" which preserve this structure in some sense; such a process is called a functor.

Diagram chasing is a visual method of arguing with abstract "arrows" joined in diagrams. Functors are represented by arrows between categories, subject to specific defining commutativity conditions. Functors can define (construct) categorical diagrams and sequences (viz. Mitchell, 1965)^{[ citation needed ]}. A functor associates to every object of one category an object of another category, and to every morphism in the first category a morphism in the second.

As a result, this defines a category *of categories and functors* – the objects are categories, and the morphisms (between categories) are functors.

Studying categories and functors is not just studying a class of mathematical structures and the morphisms between them but rather the *relationships between various classes of mathematical structures*. This fundamental idea first surfaced in algebraic topology. Difficult *topological* questions can be translated into *algebraic* questions which are often easier to solve. Basic constructions, such as the fundamental group or the fundamental groupoid of a topological space, can be expressed as functors to the category of groupoids in this way, and the concept is pervasive in algebra and its applications.

Abstracting yet again, some diagrammatic and/or sequential constructions are often "naturally related" – a vague notion, at first sight. This leads to the clarifying concept of natural transformation, a way to "map" one functor to another. Many important constructions in mathematics can be studied in this context. "Naturality" is a principle, like general covariance in physics, that cuts deeper than is initially apparent. An arrow between two functors is a natural transformation when it is subject to certain naturality or commutativity conditions.

Functors and natural transformations ('naturality') are the key concepts in category theory.^{ [5] }

A *category**C* consists of the following three mathematical entities:

- A class ob(
*C*), whose elements are called*objects*; - A class hom(
*C*), whose elements are called morphisms or maps or*arrows*. Each morphismhas a**f***source object*and**a***target object*.**b**

The expression*f*:*a*→*b*, would be verbally stated as "*f*is a morphism from*a*to*b*".

The expression**hom(**– alternatively expressed as*a*,*b*)**hom**,_{C}(*a*,*b*)**mor(**, or*a*,*b*)– denotes the*C*(*a*,*b*)*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 ∘ : hom(*b*,*c*) × hom(*a*,*b*) → hom(*a*,*c*). The composition of*f*:*a*→*b*and*g*:*b*→*c*is written as*g*∘*f*or*gf*,^{ [lower-alpha 1] }governed by two axioms:- Associativity: If
*f*:*a*→*b*,*g*:*b*→*c*and*h*:*c*→*d*then*h*∘ (*g*∘*f*) = (*h*∘*g*) ∘*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}∘*f*=*f*=*f*∘ 1_{a}.

- Associativity: If

- 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.

Relations among morphisms (such as *fg* = *h*) 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 *f* : *a* → *b* is a:

- monomorphism (or
*monic*) if*f*∘*g*_{1}=*f*∘*g*_{2}implies*g*_{1}=*g*_{2}for all morphisms*g*_{1},*g*:_{2}*x*→*a*. - epimorphism (or
*epic*) if*g*_{1}∘*f*=*g*_{2}∘*f*implies*g*=_{1}*g*for all morphisms_{2}*g*,_{1}*g*:_{2}*b*→*x*. *bimorphism*if*f*is both epic and monic.- isomorphism if there exists a morphism
*g*:*b*→*a*such that*f*∘*g*= 1_{b}and*g*∘*f*= 1_{a}.^{ [lower-alpha 2] } - endomorphism if
*a*=*b*. 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*g*:*b*→*a*with*f*∘*g*= 1_{b}. - section if a left inverse of
*f*exists, i.e. if there exists a morphism*g*:*b*→*a*with*g*∘*f*= 1_{a}.

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 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 *F* : *C* → *D*, consists of:

- for each object
*x*in*C*, an object*F*(*x*) in*D*; and - for each morphism
*f*:*x*→*y*in*C*, a morphism*F*(*f*) :*F*(*x*) →*F*(*y*),

such that the following two properties hold:

- For every object
*x*in*C*,*F*(1_{x}) = 1_{F(x)}; - For all morphisms
*f*:*x*→*y*and*g*:*y*→*z*,*F*(*g*∘*f*) =*F*(*g*) ∘*F*(*f*).

A **contravariant** functor *F*: *C* → *D* is like a covariant functor, except that it "turns morphisms around" ("reverses all the arrows"). More specifically, every morphism *f* : *x* → *y* in *C* must be assigned to a morphism *F*(*f*) : *F*(*y*) → *F*(*x*) in *D*. In other words, a contravariant functor acts as a covariant functor from the opposite category *C*^{op} to *D*.

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 η_{X} : *F*(*X*) → *G*(*X*) in *D* such that for every morphism *f* : *X* → *Y* in *C*, we have η_{Y} ∘ *F*(*f*) = *G*(*f*) ∘ η_{X}; 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*.

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*.

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.

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.

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 numbers *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, see John Baez, 'A Tale of *n*-categories' (1996).

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It should be observed first that the whole concept of a category is essentially an auxiliary one; our basic concepts are essentially those of a functor and of a natural transformation [...]

In 1942–45, Samuel Eilenberg and Saunders Mac Lane introduced categories, functors, and natural transformations as part of their work in topology, especially algebraic topology. Their work was an important part of the transition from intuitive and geometric homology to homological algebra. Eilenberg and Mac Lane later wrote that their goal was to understand natural transformations. That required defining functors, which required 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;^{[ citation needed ]} Noether realized that understanding a type of mathematical structure requires understanding the processes that preserve that structure (homomorphisms).^{[ citation needed ]} 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, 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 and semantics (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.

Categorical logic is now a well-defined field based on type theory for intuitionistic logics, with applications in functional programming 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 has shown a link between Feynman diagrams in physics and monoidal categories.^{ [7] } 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).

- ↑ Some authors compose in the opposite order, writing
*fg*or*f*∘*g*for*g*∘*f*. Computer scientists using category theory very commonly write*f*;*g*for*g*∘*f* - ↑ Note that a morphism that is both epic and monic is not necessarily an isomorphism! An elementary counterexample: in the category consisting of two objects
*A*and*B*, the identity morphisms, and a single morphism*f*from*A*to*B*,*f*is both epic and monic but is not an isomorphism.

In mathematics, specifically category theory, a **functor** is a mapping between categories. Functors were first considered in algebraic topology, where algebraic objects are associated to topological spaces, and maps between these algebraic objects are associated to continuous maps between spaces. Nowadays, functors are used throughout modern mathematics to relate various categories. Thus, functors are important in all areas within mathematics to which category theory is applied.

In category theory, a branch of mathematics, a **universal property** is an important property which is satisfied by a **universal morphism**. Universal morphisms can also be thought of more abstractly as initial or terminal objects of a comma category. Universal properties occur almost everywhere in mathematics, and hence the precise category theoretic concept helps point out similarities between different branches of mathematics, some of which may even seem unrelated.

In category theory, a branch of mathematics, a **natural transformation** provides a way of transforming one functor into another while respecting the internal structure of the categories involved. Hence, a natural transformation can be considered to be a "morphism of functors". Indeed, this intuition can be formalized to define so-called functor categories. Natural transformations are, after categories and functors, one of the most fundamental notions of category theory and consequently appear in the majority of its applications.

In mathematics, a **category** is a collection of "objects" that are linked by "arrows". A category has two basic properties: the ability to compose the arrows associatively and the existence of an identity arrow for each object. A simple example is the category of sets, whose objects are sets and whose arrows are functions.

In mathematics, specifically category theory, **adjunction** is a relationship that two functors may have. Two functors that stand in this relationship are known as **adjoint functors**, one being the **left adjoint** and the other the **right adjoint**. Pairs of adjoint functors are ubiquitous in mathematics and often arise from constructions of "optimal solutions" to certain problems, such as the construction of a free group on a set in algebra, or the construction of the Stone–Čech compactification of a topological space in topology.

**Homological algebra** is the branch of mathematics that studies homology in a general algebraic setting. It is a relatively young discipline, whose origins can be traced to investigations in combinatorial topology and abstract algebra at the end of the 19th century, chiefly by Henri Poincaré and David Hilbert.

In category theory, a **category is Cartesian closed** if, roughly speaking, any morphism defined on a product of two objects can be naturally identified with a morphism defined on one of the factors. These categories are particularly important in mathematical logic and the theory of programming, in that their internal language is the simply typed lambda calculus. They are generalized by closed monoidal categories, whose internal language, linear type systems, are suitable for both quantum and classical computation.

In mathematics, a **monoidal category** is a category equipped with a bifunctor

The following outline is provided as an overview of and guide to category theory, the area of study in mathematics that examines in an abstract way the properties of particular mathematical concepts, by formalising them as collections of *objects* and *arrows*, where these collections satisfy certain basic conditions. Many significant areas of mathematics can be formalised as categories, and the use of category theory allows many intricate and subtle mathematical results in these fields to be stated, and proved, in a much simpler way than without the use of categories.

In category theory, a branch of mathematics, a **monad** is an endofunctor, together with two natural transformations required to fulfill certain coherence conditions. Monads are used in the theory of pairs of adjoint functors, and they generalize closure operators on partially ordered sets to arbitrary categories.

In category theory, an abstract branch of mathematics, an **equivalence of categories** is a relation between two categories that establishes that these categories are "essentially the same". There are numerous examples of categorical equivalences from many areas of mathematics. Establishing an equivalence involves demonstrating strong similarities between the mathematical structures concerned. In some cases, these structures may appear to be unrelated at a superficial or intuitive level, making the notion fairly powerful: it creates the opportunity to "translate" theorems between different kinds of mathematical structures, knowing that the essential meaning of those theorems is preserved under the translation.

In mathematics, particularly category theory, a **representable functor** is a certain functor from an arbitrary category into the category of sets. Such functors give representations of an abstract category in terms of known structures allowing one to utilize, as much as possible, knowledge about the category of sets in other settings.

In mathematics, **Brown's representability theorem** in homotopy theory gives necessary and sufficient conditions for a contravariant functor *F* on the homotopy category *Hotc* of pointed connected CW complexes, to the category of sets **Set**, to be a representable functor.

In category theory, a branch of mathematics, a **closed category** is a special kind of category.

**Fibred categories** are abstract entities in mathematics used to provide a general framework for descent theory. They formalise the various situations in geometry and algebra in which *inverse images* of objects such as vector bundles can be defined. As an example, for each topological space there is the category of vector bundles on the space, and for every continuous map from a topological space *X* to another topological space *Y* is associated the pullback functor taking bundles on *Y* to bundles on *X*. Fibred categories formalise the system consisting of these categories and inverse image functors. Similar setups appear in various guises in mathematics, in particular in algebraic geometry, which is the context in which fibred categories originally appeared. Fibered categories are used to define stacks, which are fibered categories with "descent". Fibrations also play an important role in categorical semantics of type theory, and in particular that of dependent type theories.

This is a glossary of properties and concepts in category theory in mathematics.

In mathematics, **higher category theory** is the part of category theory at a *higher order*, which means that some equalities are replaced by explicit arrows in order to be able to explicitly study the structure behind those equalities. Higher category theory is often applied in algebraic topology, where one studies algebraic invariants of spaces, such as their fundamental weak ∞-groupoid.

In category theory, a branch of mathematics, a **diagram** is the categorical analogue of an indexed family in set theory. The primary difference is that in the categorical setting one has morphisms that also need indexing. An indexed family of sets is a collection of sets, indexed by a fixed set; equivalently, a *function* from a fixed index *set* to the class of *sets*. A diagram is a collection of objects and morphisms, indexed by a fixed category; equivalently, a *functor* from a fixed index *category* to some *category*.

In mathematics, especially (higher) category theory, **higher-dimensional algebra** is the study of categorified structures. It has applications in nonabelian algebraic topology, and generalizes abstract algebra.

In mathematics, a **topos** is a category that behaves like the category of sheaves of sets on a topological space. Topoi behave much like the category of sets and possess a notion of localization; they are a direct generalization of point-set topology. The **Grothendieck topoi** find applications in algebraic geometry; the more general **elementary topoi** are used in logic.

- ↑ Awodey, Steve (2010) [2006].
*Category Theory*. Oxford Logic Guides.**49**(2nd ed.). Oxford University Press. ISBN 978-0-19-923718-0. - ↑ Geroch, Robert (1985).
*Mathematical physics*([Repr.] ed.). University of Chicago Press. pp. 7. ISBN 978-0-226-28862-8.Note that theorem 3 is actually easier for categories in general than it is for the special case of sets. This phenomenon is by no means rare.

- ↑ Coecke, B., ed. (2011).
*New Structures for Physics*. Lecture Notes in Physics.**831**. Springer-Verlag. ISBN 9783642128202. - ↑ Rosen, Robert (1958). "The representation of biological systems from the standpoint of the theory of categories" (PDF).
*Bulletin of Mathematical Biophysics*.**20**(4): 317–341. doi:10.1007/BF02477890. - ↑ Mac Lane 1998 , p. 18: "As Eilenberg-Mac Lane first observed, 'category' has been defined in order to be able to define 'functor' and 'functor' has been defined in order to be able to define 'natural transformation'."
- ↑ Eilenberg, Samuel; MacLane, Saunders (1945). "General theory of natural equivalences".
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- 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.
- Hillman, Chris,
*A Categorical Primer*, CiteSeerX 10.1.1.24.3264 , a formal introduction to category theory. - Adamek, J.; Herrlich, H.; Stecker, G. "Abstract and Concrete Categories-The Joy of Cats" (PDF).
- "Category Theory" entry by Jean-Pierre Marquis in the
*Stanford Encyclopedia of Philosophy*, with an extensive bibliography. - List of academic conferences on category theory
- Baez, John (1996). "The Tale of
*n*-categories". — 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.
- The catsters's channel on YouTube, a channel about category theory.
- Category theory at PlanetMath ..
- 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.
- Category Theory for the Sciences, 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.

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