Homotopy theory explained

In mathematics, homotopy theory is a systematic study of situations in which maps can come with homotopies between them. It originated as a topic in algebraic topology, but nowadays is learned 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

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.

Homotopy

See main article: Homotopy. Let I denote the unit interval. A family of maps indexed by I,

h _{t} : X \to Y is called a homotopy from h _{0} to h _{1} if h : I x X \to Y, (t, x) \mapsto h _{t(x)} is a map (e.g., it must be a continuous function). When X, Y are pointed spaces, the h _{t} are required to preserve the basepoints. A homotopy can be shown to be an equivalence relation. Given a pointed space X and an integer n \ge 1 , let \pi _{n(X)} = [S ^{n,} X] _{*} be the homotopy classes of based maps S ^{n} \to X from a (pointed) n-sphere S ^{n} to X. As it turns out, \pi _{n(X)} are groups; in particular, \pi _{1(X)} 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.

Cofibration and fibration

A map

f: A \to X is called a cofibration if given (1) a map h _{0} : X \to Z and (2) a homotopy g _{t} : A \to Z , there exists a homotopy h _{t} : X \to Z that extends h _{0} and such that h _{t} \circ f = g _{t} . In 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 (X, A) ; 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

p : X \to B is a fibration if given (1) a map Z \to X and (2) a homotopy g _{t} : Z \to B , there exists a homotopy h _{t:} Z \to X such that h _{0} is the given one and p \circ h _{t} = g _{t} . A basic example is a covering map (in fact, a fibration is a generalization of a covering map). If E 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 p: E \to X is an example of a fibration.

Classifying spaces and homotopy operations

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

BG such that, for each space X,[X, BG] = / ~ , [f] \mapsto f ^{*} EG where

• the left-hand side is the set of homotopy classes of maps

X \to BG ,

• ~ refers isomorphism of bundles, and
• = is given by pulling-back the distinguished bundle

EG on BG (called universal bundle) along a map X \to BG .Brown's representability theorem guarantees the existence of classifying spaces.

Spectrum and generalized cohomology

See main article: Spectrum (algebraic topology).

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

Z ),[X, K(A, n)] = \operatorname{H} ^{n(X;} A) where K(A, n) 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:

S ^{0} \to S ^{1} \to S ^{2} \to …

Key theorems

• 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

Obstruction theory and characteristic class

See also: Characteristic class, Postnikov tower, Whitehead torsion

Localization and completion of a space

See main article: Localization of a topological space.

Specific theories

There are several specific theories

• simple homotopy theory
• stable homotopy theory
• chromatic homotopy theory
• rational homotopy theory
• p-adic homotopy theory
• equivariant homotopy theory

Homotopy hypothesis

See main article: Homotopy hypothesis.

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.

Abstract homotopy theory

Concepts

• fiber sequence
• cofiber sequence

Model categories

See main article: Model category.

Simplicial homotopy theory

• Simplicial homotopy

See also

• Highly structured ring spectrum
• Homotopy type theory
• Pursuing Stacks
• Shape theory

References

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

Further reading

• Cisinski's notes
• http://ncatlab.org/nlab/files/Abstract-Homotopy.pdf
• Math 527 - Homotopy Theory Spring 2013, Section F1, lectures by Martin Frankland

External links

Web site: homotopy theory . ncatlab.org.