In mathematics, **homotopy theory** is a systematic study of situations in which maps come with homotopies between them. It originated as a topic in algebraic topology but nowadays is studied as an independent discipline. Besides algebraic topology, the theory has also been used in other areas of mathematics such as algebraic geometry (e.g., A¹ homotopy theory) and category theory (specifically the study of higher categories).

- Concepts
- Spaces and maps
- Homotopy
- Cofibration and fibration
- Classifying spaces and homotopy operations
- Spectrum and generalized cohomology
- Key theorems
- Obstruction theory and characteristic class
- Localization and completion of a space
- Specific theories
- Homotopy hypothesis
- Abstract homotopy theory
- Concepts 2
- Model categories
- Simplicial homotopy theory
- See also
- References
- Further reading
- External links

In homotopy theory and algebraic topology, the word "space" denotes a topological space. In order to avoid pathologies, one rarely works with arbitrary spaces; instead, one requires spaces to meet extra constraints, such as being compactly generated, or Hausdorff, or a CW complex.

In the same vein as above, a "map" is a continuous function, possibly with some extra constraints.

Often, one works with a pointed space -- that is, a space with a "distinguished point", called a basepoint. A pointed map is then a map which preserves basepoints; that is, it sends the basepoint of the domain to that of the codomain. In contrast, a free map is one which needn't preserve basepoints.

Let *I* denote the unit interval. A family of maps indexed by *I*, is called a homotopy from to if is a map (e.g., it must be a continuous function). When *X*, *Y* are pointed spaces, the are required to preserve the basepoints. A homotopy can be shown to be an equivalence relation. Given a pointed space *X* and an integer , let be the homotopy classes of based maps from a (pointed) *n*-sphere to *X*. As it turns out, are groups; in particular, is called the fundamental group of *X*.

If one prefers to work with a space instead of a pointed space, there is the notion of a fundamental groupoid (and higher variants): by definition, the fundamental groupoid of a space *X* is the category where the objects are the points of *X* and the morphisms are paths.

A map is called a cofibration if given (1) a map and (2) a homotopy , there exists a homotopy that extends and such that . To some loose sense, it is an analog of the defining diagram of an injective module in abstract algebra. The most basic example is a CW pair ; since many work only with CW complexes, the notion of a cofibration is often implicit.

A fibration in the sense of Serre is the dual notion of a cofibration: that is, a map is a fibration if given (1) a map and (2) a homotopy , there exists a homotopy such that is the given one and . A basic example is a covering map (in fact, a fibration is a generalization of a covering map). If is a principal *G*-bundle, that is, a space with a free and transitive (topological) group action of a (topological) group, then the projection map is an example of a fibration.

Given a topological group *G*, the classifying space for principal *G*-bundles ("the" up to equivalence) is a space such that, for each space *X*,

- { principal
*G*-bundle on*X*} / ~

where

- the left-hand side is the set of homotopy classes of maps ,
- ~ refers isomorphism of bundles, and
- = is given by pulling-back the distinguished bundle on (called universal bundle) along a map .

Brown's representability theorem guarantees the existence of classifying spaces.

The idea that a classifying space classifies principal bundles can be pushed further. For example, one might try to classify cohomology classes: given an abelian group *A* (such as ),

where is the Eilenberg–MacLane space. The above equation leads to the notion of a generalized cohomology theory; i.e., a contravariant functor from the category of spaces to the category of abelian groups that satisfies the axioms generalizing ordinary cohomology theory. As it turns out, such a functor may not be representable by a space but it can always be represented by a sequence of (pointed) spaces with structure maps called a spectrum. In other words, to give a generalized cohomology theory is to give a spectrum.

A basic example of a spectrum is a sphere spectrum:

- Seifert–van Kampen theorem
- Homotopy excision theorem
- Freudenthal suspension theorem (a corollary of the excision theorem)
- Landweber exact functor theorem
- Dold–Kan correspondence
- Eckmann–Hilton argument - this shows for instance higher homotopy groups are abelian.
- Universal coefficient theorem

See also: Characteristic class, Postnikov tower, Whitehead torsion

There are several specific theories

One of the basic questions in the foundations of homotopy theory is the nature of a space. The homotopy hypothesis asks whether a space is something fundamentally algebraic.

**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 up to homotopy equivalence.

In mathematics, specifically in homology theory and algebraic topology, **cohomology** is a general term for a sequence of abelian groups associated to a topological space, often defined from a cochain complex. Cohomology can be viewed as a method of assigning richer algebraic invariants to a space than homology. Some versions of cohomology arise by dualizing the construction of homology. In other words, cochains are functions on the group of chains in homology theory.

In mathematics, **homotopy groups** are used in algebraic topology to classify topological spaces. The first and simplest homotopy group is the fundamental group, which records information about loops in a space. Intuitively, homotopy groups record information about the basic shape, or *holes*, of a topological space.

In mathematics, a **characteristic class** is a way of associating to each principal bundle of *X* a cohomology class of *X*. The cohomology class measures the extent the bundle is "twisted" and whether it possesses sections. Characteristic classes are global invariants that measure the deviation of a local product structure from a global product structure. They are one of the unifying geometric concepts in algebraic topology, differential geometry, and algebraic geometry.

**Algebraic K-theory** is a subject area in mathematics with connections to geometry, topology, ring theory, and number theory. Geometric, algebraic, and arithmetic objects are assigned objects called

In mathematics, a **gerbe** is a construct in homological algebra and topology. Gerbes were introduced by Jean Giraud following ideas of Alexandre Grothendieck as a tool for non-commutative cohomology in degree 2. They can be seen as an analogue of fibre bundles where the fibre is the classifying stack of a group. Gerbes provide a convenient, if highly abstract, language for dealing with many types of deformation questions especially in modern algebraic geometry. In addition, special cases of gerbes have been used more recently in differential topology and differential geometry to give alternative descriptions to certain cohomology classes and additional structures attached to them.

In mathematics, specifically in homotopy theory, a **classifying space***BG* of a topological group *G* is the quotient of a weakly contractible space *EG* by a proper free action of *G*. It has the property that any *G* principal bundle over a paracompact manifold is isomorphic to a pullback of the principal bundle *EG* → *BG*. As explained later, this means that classifying spaces represent a set-valued functor on the homotopy category of topological spaces. The term classifying space can also be used for spaces that represent a set-valued functor on the category of topological spaces, such as Sierpiński space. This notion is generalized by the notion of classifying topos. However, the rest of this article discusses the more commonly used notion of classifying space up to homotopy.

In algebraic topology, a branch of mathematics, a **spectrum** is an object representing a generalized cohomology theory. This means given a cohomology theory

In mathematics, specifically algebraic topology, an **Eilenberg–MacLane space** is a topological space with a single nontrivial homotopy group. As such, an Eilenberg–MacLane space is a special kind of topological space that can be regarded as a building block for homotopy theory; general topological spaces can be constructed from these via the Postnikov system. These spaces are important in many contexts in algebraic topology, including constructions of spaces, computations of homotopy groups of spheres, and definition of cohomology operations. The name is for Samuel Eilenberg and Saunders Mac Lane, who introduced such spaces in the late 1940s.

In mathematics, in particular homotopy theory, a continuous mapping

In mathematics, the **classifying space for the unitary group** U(*n*) is a space BU(*n*) together with a universal bundle EU(*n*) such that any hermitian bundle on a paracompact space *X* is the pull-back of EU(*n*) by a map *X* → BU(*n*) unique up to homotopy.

In mathematics, **topological K-theory** is a branch of algebraic topology. It was founded to study vector bundles on topological spaces, by means of ideas now recognised as (general) K-theory that were introduced by Alexander Grothendieck. The early work on topological K-theory is due to Michael Atiyah and Friedrich Hirzebruch.

In mathematics, **equivariant cohomology** is a cohomology theory from algebraic topology which applies to topological spaces with a *group action*. It can be viewed as a common generalization of group cohomology and an ordinary cohomology theory. Specifically, the equivariant cohomology ring of a space with action of a topological group is defined as the ordinary cohomology ring with coefficient ring of the homotopy quotient :

In mathematics, more specifically algebraic topology, a pair is shorthand for an inclusion of topological spaces . Sometimes is assumed to be a cofibration. A morphism from to is given by two maps and such that .

In mathematics, a **normal map** is a concept in geometric topology due to William Browder which is of fundamental importance in surgery theory. Given a Poincaré complex *X*, a normal map on *X* endows the space, roughly speaking, with some of the homotopy-theoretic global structure of a closed manifold. In particular, *X* has a good candidate for a stable normal bundle and a Thom collapse map, which is equivalent to there being a map from a manifold *M* to *X* matching the fundamental classes and preserving normal bundle information. If the dimension of *X* is 5 there is then only the algebraic topology surgery obstruction due to C. T. C. Wall to *X* actually being homotopy equivalent to a closed manifold. Normal maps also apply to the study of the uniqueness of manifold structures within a homotopy type, which was pioneered by Sergei Novikov.

In mathematics, a **2-group**, or **2-dimensional higher group**, is a certain combination of group and groupoid. The 2-groups are part of a larger hierarchy of *n*-groups. In some of the literature, 2-groups are also called **gr-categories** or **groupal groupoids**.

In mathematics, a **weak equivalence** is a notion from homotopy theory that in some sense identifies objects that have the same "shape". This notion is formalized in the axiomatic definition of a model category.

This is a glossary of properties and concepts in algebraic topology in mathematics.

In Mathematics, an **Abelian 2-group** is a higher dimensional analogue of an Abelian group, in the sense of higher algebra, which were originally introduced by Alexander Grothendieck while studying abstract structures surrounding Abelian varieties and Picard groups. More concretely, they are given by groupoids which have a bifunctor which acts formally like the addition an Abelian group. Namely, the bifunctor has a notion of commutativity, associativity, and an identity structure. Although this seems like a rather lofty and abstract structure, there are several examples of Abelian 2-groups. In fact, some of which provide prototypes for more complex examples of higher algebraic structures, such as Abelian n-groups.

- May, J. A Concise Course in Algebraic Topology
- George William Whitehead (1978).
*Elements of homotopy theory*. Graduate Texts in Mathematics.**61**(3rd ed.). New York-Berlin: Springer-Verlag. pp. xxi+744. ISBN 978-0-387-90336-1. MR 0516508 . Retrieved September 6, 2011. - Ronald Brown,
*Topology and groupoids*(2006) Booksurge LLC ISBN 1-4196-2722-8.

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