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*.

The universal functor of a diagram is the diagonal functor; its right adjoint is the limit of the diagram and its left adjoint is the colimit.^{ [1] } The natural transformation from the diagonal functor to some arbitrary diagram is called a cone.

Formally, a **diagram** of type *J* in a category *C* is a (covariant) functor

The category *J* is called the **index category** or the **scheme** of the diagram *D*; the functor is sometimes called a ** J-shaped diagram**.

Although, technically, there is no difference between an individual *diagram* and a *functor* or between a *scheme* and a *category*, the change in terminology reflects a change in perspective, just as in the set theoretic case: one fixes the index category, and allows the functor (and, secondarily, the target category) to vary.

One is most often interested in the case where the scheme *J* is a small or even finite category. A diagram is said to be **small** or **finite** whenever *J* is.

A morphism of diagrams of type *J* in a category *C* is a natural transformation between functors. One can then interpret the **category of diagrams** of type *J* in *C* as the functor category *C*^{J}, and a diagram is then an object in this category.

- Given any object
*A*in*C*, one has the**constant diagram**, which is the diagram that maps all objects in*J*to*A*, and all morphisms of*J*to the identity morphism on*A*. Notationally, one often uses an underbar to denote the constant diagram: thus, for any object in*C*, one has the constant diagram . - If
*J*is a (small) discrete category, then a diagram of type*J*is essentially just an indexed family of objects in*C*(indexed by*J*). When used in the construction of the limit, the result is the product; for the colimit, one gets the coproduct. So, for example, when*J*is the discrete category with two objects, the resulting limit is just the binary product. - If
*J*= −1 ← 0 → +1, then a diagram of type*J*(*A*←*B*→*C*) is a span, and its colimit is a pushout. If one were to "forget" that the diagram had object*B*and the two arrows*B*→*A*,*B*→*C*, the resulting diagram would simply be the discrete category with the two objects*A*and*C*, and the colimit would simply be the binary coproduct. Thus, this example shows an important way in which the idea of the diagram generalizes that of the index set in set theory: by including the morphisms*B*→*A*,*B*→*C*, one discovers additional structure in constructions built from the diagram, structure that would not be evident if one only had an index set with no relations between the objects in the index. - Dual to the above, if
*J*= −1 → 0 ← +1, then a diagram of type*J*(*A*→*B*←*C*) is a cospan, and its limit is a pullback. - The index is called "two parallel morphisms", or sometimes the free quiver or the walking quiver. A diagram of type is then a quiver; its limit is an equalizer, and its colimit is a coequalizer.
- If
*J*is a poset category, then a diagram of type*J*is a family of objects*D*_{i}together with a unique morphism*f*_{ij}:*D*_{i}→*D*_{j}whenever*i*≤*j*. If*J*is directed then a diagram of type*J*is called a direct system of objects and morphisms. If the diagram is contravariant then it is called an inverse system.

A cone with vertex *N* of a diagram *D* : *J* → *C* is a morphism from the constant diagram Δ(*N*) to *D*. The constant diagram is the diagram which sends every object of *J* to an object *N* of *C* and every morphism to the identity morphism on *N*.

The limit of a diagram *D* is a universal cone to *D*. That is, a cone through which all other cones uniquely factor. If the limit exists in a category *C* for all diagrams of type *J* one obtains a functor

lim : *C*^{J}→*C*

which sends each diagram to its limit.

Dually, the colimit of diagram *D* is a universal cone from *D*. If the colimit exists for all diagrams of type *J* one has a functor

colim : *C*^{J}→*C*

which sends each diagram to its colimit.

Diagrams and functor categories are often visualized by commutative diagrams, particularly if the index category is a finite poset category with few elements: one draws a commutative diagram with a node for every object in the index category, and an arrow for a generating set of morphisms, omitting identity maps and morphisms that can be expressed as compositions. The commutativity corresponds to the uniqueness of a map between two objects in a poset category. Conversely, every commutative diagram represents a diagram (a functor from a poset index category) in this way.

Not every diagram commutes, as not every index category is a poset category: most simply, the diagram of a single object with an endomorphism (), or with two parallel arrows (; ) need not commute. Further, diagrams may be impossible to draw (because they are infinite) or simply messy (because there are too many objects or morphisms); however, schematic commutative diagrams (for subcategories of the index category, or with ellipses, such as for a directed system) are used to clarify such complex diagrams.

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 mathematics, the **inverse limit** is a construction that allows one to "glue together" several related objects, the precise manner of the gluing process being specified by morphisms between the objects. Inverse limits can be defined in any category, and they are a special case of the concept of a limit in category theory.

In category theory, a branch of mathematics, the abstract notion of a **limit** captures the essential properties of universal constructions such as products, pullbacks and inverse limits. The dual notion of a **colimit** generalizes constructions such as disjoint unions, direct sums, coproducts, pushouts and direct limits.

In mathematics, an **abelian category** is a category in which morphisms and objects can be added and in which kernels and cokernels exist and have desirable properties. The motivating prototypical example of an abelian category is the category of abelian groups, **Ab**. The theory originated in an effort to unify several cohomology theories by Alexander Grothendieck and independently in the slightly earlier work of David Buchsbaum. Abelian categories are very *stable* categories; for example they are regular and they satisfy the snake lemma. The class of abelian categories is closed under several categorical constructions, for example, the category of chain complexes of an abelian category, or the category of functors from a small category to an abelian category are abelian as well. These stability properties make them inevitable in homological algebra and beyond; the theory has major applications in algebraic geometry, cohomology and pure category theory. Abelian categories are named after Niels Henrik Abel.

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.

In category theory, a branch of mathematics, an **initial object** of a category C is an object I in C such that for every object X in C, there exists precisely one morphism *I* → *X*.

In mathematics, and especially in category theory, a **commutative diagram** is a diagram such that all directed paths in the diagram with the same start and endpoints lead to the same result. It is said that commutative diagrams play the role in category theory that equations play in algebra.

In category theory, the **product** of two objects in a category is a notion designed to capture the essence behind constructions in other areas of mathematics such as the Cartesian product of sets, the direct product of groups or rings, and the product of topological spaces. Essentially, the product of a family of objects is the "most general" object which admits a morphism to each of the given objects.

In category theory, the **coproduct**, or **categorical sum**, is a construction which includes as examples the disjoint union of sets and of topological spaces, the free product of groups, and the direct sum of modules and vector spaces. The coproduct of a family of objects is essentially the "least specific" object to which each object in the family admits a morphism. It is the category-theoretic dual notion to the categorical product, which means the definition is the same as the product but with all arrows reversed. Despite this seemingly innocuous change in the name and notation, coproducts can be and typically are dramatically different from products.

In mathematics, a **quiver** is a directed graph where loops and multiple arrows between two vertices are allowed, i.e. a multidigraph. They are commonly used in representation theory: a representation *V* of a quiver assigns a vector space *V*(*x*) to each vertex *x* of the quiver and a linear map *V*(*a*) to each arrow *a*.

In category theory, a branch of mathematics, a **pushout** is the colimit of a diagram consisting of two morphisms *f* : *Z* → *X* and *g* : *Z* → *Y* with a common domain. The pushout consists of an object *P* along with two morphisms *X* → *P* and *Y* → *P* that complete a commutative square with the two given morphisms *f* and *g*. In fact, the defining universal property of the pushout essentially says that the pushout is the "most general" way to complete this commutative square. Common notations for the pushout are and .

In mathematics, in the field of category theory, a **discrete category** is a category whose only morphisms are the identity morphisms:

In mathematics, a **triangulated category** is a category with the additional structure of a "translation functor" and a class of "exact triangles". Prominent examples are the derived category of an abelian category, as well as the stable homotopy category. The exact triangles generalize the short exact sequences in an abelian category, as well as fiber sequences and cofiber sequences in topology.

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

In mathematics, specifically in category theory, hom-sets, i.e. sets of morphisms between objects, give rise to important functors to the category of sets. These functors are called **hom-functors** and have numerous applications in category theory and other branches of mathematics.

In category theory, a branch of mathematics, the **diagonal functor** is given by , which maps objects as well as morphisms. This functor can be employed to give a succinct alternate description of the product of objects *within* the category : a product is a universal arrow from to . The arrow comprises the projection maps.

In category theory, a branch of mathematics, the **cone of a functor** is an abstract notion used to define the limit of that functor. Cones make other appearances in category theory as well.

In mathematics, **derivators** are a proposed new framework^{pg 190-195} for homological algebra giving a framework for non-abelian homological algebra and various generalisations of it. They were introduced to address the deficiencies of derived categories and provide at the same time a language for homotopical algebra.

In mathematics, the **category of rings**, denoted by **Ring**, is the category whose objects are rings and whose morphisms are ring homomorphisms. Like many categories in mathematics, the category of rings is large, meaning that the class of all rings is proper.

In mathematics, especially in algebraic topology, the **homotopy limit and colimit**^{pg 52} are variants of the notions of limit and colimit extended to the homotopy category . The main idea is this: if we have a diagram

- ↑ Mac Lane, Saunders; Moerdijk, Ieke (1992).
*Sheaves in geometry and logic a first introduction to topos theory*. New York: Springer-Verlag. pp. 20–23. ISBN 9780387977102. - ↑ May, J. P. (1999).
*A Concise Course in Algebraic Topology*(PDF). University of Chicago Press. p. 16. ISBN 0-226-51183-9.

- Adámek, Jiří; Horst Herrlich; George E. Strecker (1990).
*Abstract and Concrete Categories*(PDF). John Wiley & Sons. ISBN 0-471-60922-6. Now available as free on-line edition (4.2MB PDF). - Barr, Michael; Wells, Charles (2002).
*Toposes, Triples and Theories*(PDF). ISBN 0-387-96115-1. Revised and corrected free online version of*Grundlehren der mathematischen Wissenschaften (278)*Springer-Verlag, 1983). - diagram in
*nLab*

- Diagram Chasing at MathWorld
- WildCats is a category theory package for Mathematica. Manipulation and visualization of objects, morphisms, commutative diagrams, categories, functors, natural transformations.

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