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350px, The commutative diagram used in the proof of the five lemma. In
mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, and especially in
category theory 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. Nowadays, ca ...
, a commutative diagram is a
diagram A diagram is a symbolic representation of information using visualization techniques. Diagrams have been used since prehistoric times on walls of caves, but became more prevalent during the Enlightenment. Sometimes, the technique uses a three ...
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 In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in ...
play in
algebra Algebra () is one of the broad areas of mathematics. Roughly speaking, algebra is the study of mathematical symbols and the rules for manipulating these symbols in formulas; it is a unifying thread of almost all of mathematics. Elementary ...
.


Description

A commutative diagram often consists of three parts: *
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 ...
(also known as ''vertices'') * morphisms (also known as ''arrows'' or ''edges'') * paths or composites


Arrow symbols

In algebra texts, the type of morphism can be denoted with different arrow usages: * A monomorphism may be labeled with a \hookrightarrow or a \rightarrowtail. * An epimorphism may be labeled with a \twoheadrightarrow. * An
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 ...
may be labeled with a \overset. * The dashed arrow typically represents the claim that the indicated morphism exists (whenever the rest of the diagram holds); the arrow may be optionally labeled as \exists. ** If the morphism is in addition unique, then the dashed arrow may be labeled ! or \exists!. The meanings of different arrows are not entirely standardized: the arrows used for monomorphisms, epimorphisms, and isomorphisms are also used for injections, surjections, and
bijection In mathematics, a bijection, also known as a bijective function, one-to-one correspondence, or invertible function, is a function between the elements of two sets, where each element of one set is paired with exactly one element of the other ...
s, as well as the cofibrations, fibrations, and weak equivalences in a
model category In mathematics, particularly in homotopy theory, a model category is a category with distinguished classes of morphisms ('arrows') called ' weak equivalences', ' fibrations' and 'cofibrations' satisfying certain axioms relating them. These abstra ...
.


Verifying commutativity

Commutativity makes sense for a
polygon In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed '' polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two ...
of any finite number of sides (including just 1 or 2), and a diagram is commutative if every polygonal subdiagram is commutative. Note that a diagram may be non-commutative, i.e., the composition of different paths in the diagram may not give the same result.


Phrases

Phrases like "this commutative diagram" or "the diagram commutes" may be used.


Examples


Example 1

In the left diagram, which expresses the first isomorphism theorem, commutativity of the triangle means that f = \tilde \circ \pi. In the right diagram, commutativity of the square means h \circ f = k \circ g.


Example 2

In order for the diagram below to commute, three equalities must be satisfied: # r \circ h \circ g = H \circ G \circ l # m \circ g = G \circ l # r \circ h = H \circ m Here, since the first equality follows from the last two, it suffices to show that (2) and (3) are true in order for the diagram to commute. However, since equality (3) generally does not follow from the other two, it is generally not enough to have only equalities (1) and (2) if one were to show that the diagram commutes.


Diagram chasing

Diagram chasing (also called diagrammatic search) is a method of
mathematical proof A mathematical proof is an inferential argument for a mathematical statement, showing that the stated assumptions logically guarantee the conclusion. The argument may use other previously established statements, such as theorems; but every pr ...
used especially in homological algebra, where one establishes a property of some morphism by tracing the elements of a commutative diagram. A proof by diagram chasing typically involves the formal use of the properties of the diagram, such as injective or
surjective In mathematics, a surjective function (also known as surjection, or onto function) is a function that every element can be mapped from element so that . In other words, every element of the function's codomain is the image of one element o ...
maps, or exact sequences. A
syllogism A syllogism ( grc-gre, συλλογισμός, ''syllogismos'', 'conclusion, inference') is a kind of logical argument that applies deductive reasoning to arrive at a conclusion based on two propositions that are asserted or assumed to be tru ...
is constructed, for which the graphical display of the diagram is just a visual aid. It follows that one ends up "chasing" elements around the diagram, until the desired element or result is constructed or verified. Examples of proofs by diagram chasing include those typically given for the five lemma, the snake lemma, the zig-zag lemma, and the nine lemma.


In higher category theory

In higher category theory, one considers not only objects and arrows, but arrows between the arrows, arrows between arrows between arrows, and so on ad infinitum. For example, the category of small categories Cat is naturally a 2-category, with functors as its arrows and natural transformations as the arrows between functors. In this setting, commutative diagrams may include these higher arrows as well, which are often depicted in the following style: \Rightarrow. For example, the following (somewhat trivial) diagram depicts two categories and , together with two functors , : → and a natural transformation : ⇒ : : There are two kinds of composition in a 2-category (called vertical composition and horizontal composition), and they may also be depicted via pasting diagrams (see 2-category#Definition for examples).


Diagrams as functors

A commutative diagram in a category ''C'' can be interpreted as a functor from an index category ''J'' to ''C;'' one calls the functor a
diagram A diagram is a symbolic representation of information using visualization techniques. Diagrams have been used since prehistoric times on walls of caves, but became more prevalent during the Enlightenment. Sometimes, the technique uses a three ...
. More formally, a commutative diagram is a visualization of a diagram indexed by a poset category. Such a diagram typically includes: * a node for every object in the index category, * an arrow for a generating set of morphisms (omitting identity maps and morphisms that can be expressed as compositions), * the commutativity of the diagram (the equality of different compositions of maps between two objects), corresponding to the uniqueness of a map between two objects in a poset category. Conversely, given a commutative diagram, it defines a poset category, where: * the objects are the nodes, * there is a morphism between any two objects if and only if there is a (directed) path between the nodes, * with the relation that this morphism is unique (any composition of maps is defined by its domain and target: this is the commutativity axiom). However, not every diagram commutes (the notion of diagram strictly generalizes commutative diagram). As a simple example, the diagram of a single object with an endomorphism (f\colon X \to X), or with two parallel arrows (\bullet \rightrightarrows \bullet, that is, f,g\colon X \to Y, sometimes called the free quiver), as used in the definition of equalizer need not commute. Further, diagrams may be messy or impossible to draw, when the number of objects or morphisms is large (or even infinite).


See also

* Mathematical diagram


References


Bibliography

* Now available as free on-line edition (4.2MB PDF). * Revised and corrected free online version of ''Grundlehren der mathematischen Wissenschaften (278)'' Springer-Verlag, 1983).


External links


Diagram Chasing
at MathWorld
WildCats
is a category theory package for Mathematica. Manipulation and visualization of objects, morphisms, categories, functors, natural transformations. {{Category theory Homological algebra Category theory Mathematical proofs Mathematical terminology Diagrams