Homotopy theory

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.

Applications to other fields of mathematics

Besides algebraic topology, the theory has also been used in other areas of mathematics such as:

• Algebraic geometry (e.g., A¹ homotopy theory)
• 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.

The Cartesian product of two pointed spaces $X,Y$ are not naturally pointed. A substitute is the smash product $X\wedge Y$ which is characterized by the adjoint relation

$\operatorname {Map} (X\wedge Y,Z)=\operatorname {Map} (X,\operatorname {Map} (Y,Z))$,

that is, a smash product is an analog of a tensor product in abstract algebra (see tensor-hom adjunction.)

Homotopy

Main article: Homotopy

Let I denote the unit interval $[0,1]$. A map

$h:X\times I\to Y$

is called a homotopy from the map $h_{0}$ to the map $h_{1}$, where $h_{t}(x)=h(x,t)$. Intuitively, we may think of $h$ as a path from the map $h_{0}$ to the map $h_{1}$. Indeed, a homotopy can be shown to be an equivalence relation. When X, Y are pointed spaces, the maps $h_{t}$ are required to preserve the basepoint.

Given a pointed space X and an integer $n\geq 0$, 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, for $n>0$, $\pi _{n}(X)$ are groups called homotopy groups; in particular, $\pi _{1}(X)$ is called the fundamental group of X. Every group is the fundamental group of some space.

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 $f$ is called a homotopy equivalence if there is another map $g$ such that $f\circ g$ and $g\circ f$ are both homotopic to the identities. Two spaces are said to be homotopy equivalent if there is a homotopy equivalence between them. A homotopy equivalence class of spaces is then called a homotopy type. There is a weaker notion: a map $f:X\to Y$ is said to be a weak homotopy equivalence if $f_{*}:\pi _{n}(X)\to \pi _{n}(Y)$ is an isomorphism for each $n\geq 0$ and each choice of a base point. (Note the existence of such a map is more strict than saying that $X,Y$ have isomorphic homotopy set/groups.) In general, a homotopy equivalence is a weak homotopy equivalence but the converse need not be true.

CW complex

Main article: CW complex

A CW complex is a space that has a filtration $X\supset \cdots \supset X^{n}\supset X^{n-1}\supset \cdots \supset X^{0}$ whose union is $X$ and such that

1. $X^{0}$ is a discrete space, called the set of 0-cells (vertices) in $X$.
2. Each $X^{n}$ is obtained by attaching several n-disks, n-cells, to $X^{n-1}$ via maps $S^{n-1}\to X^{n-1}$; i.e., the boundary of an n-disk is identified with the image of $S^{n-1}$ in $X^{n-1}$.
3. A subset $U$ is open if and only if $U\cap X^{n}$ is open for each $n$.

For example, a sphere $S^{n}$ has two cells: one 0-cell and one $n$-cell, since $S^{n}$ can be obtained by collapsing the boundary $S^{n-1}$ of the n-disk to a point. Also, every compact manifold has the homotopy type of a CW complex.

Remarkably, Whitehead's theorem says that for CW complexes, a weak homotopy equivalence and a homotopy equivalence are the same thing.

Another important result is the approximation theorem. First, the homotopy category of spaces is the category where an object is a space but a morphism is the homotopy class of a map. Then

CW approximation —  There exist a functor (called the CW approximation functor)

$\Theta :\operatorname {Ho} ({\textrm {spaces}})\to \operatorname {Ho} ({\textrm {CW}})$

from the homotopy category of spaces to the homotopy category of CW complexes as well as a natural transformation

$\theta :i\circ \Theta \to \operatorname {Id} ,$

where $i:\operatorname {Ho} ({\textrm {CW}})\hookrightarrow \operatorname {Ho} ({\textrm {spaces}})$, such that each $\theta _{X}:i(\Theta (X))\to X$ is a weak homotopy equivalence.

Similar statements also hold for pairs and triads.

The above theorem justifies a common habit of working only with CW complexes. For example, given a space $X$, one can just define the homology of $X$ to the homology of the CW approximation of $X$ (the cell structure of a CW complex determines the natural homology, the cellular homology and that can be taken to be the homology of the complex.)

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 $g_{t}:A\to Z$

such that $h_{0}\circ f=g_{0}$, there exists a homotopy $h_{t}:X\to Z$ that extends $h_{0}$ and such that $h_{t}\circ f=g_{t}$. An example is a neighborhood deformation retract; that is, $X$ contains a mapping cylinder neighborhood of a closed subspace $A$ and $f$ the inclusion (e.g., a tubular neighborhood of a closed submanifold). In fact, a cofibration can be characterized as a neighborhood deformation retract pair. Another basic example is a CW pair $(X,A)$; i.e., $A$ is a subcomplex of $X$. Many often work only with CW complexes and the notion of a cofibration there is then 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 $h_{0}:Z\to X$ and (2) a homotopy $g_{t}:Z\to B$ such that $p\circ h_{0}=g_{0}$, there exists a homotopy $h_{t}:Z\to X$ that extends $h_{0}$ and such that $p\circ h_{t}=g_{t}$. 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.

Loop and suspension

On the category of pointed spaces, there are two important functors: the loop functor $\Omega$ and the suspension functor $\Sigma$, which are in the adjoint relation. Precisely, they are defined as

• $\Omega X=\operatorname {Map} (S^{1},X)$, and
• $\Sigma X=X\wedge S^{1}$.

Because of the adjoint relation between a smash product and a mapping space, we have:

$\operatorname {Map} (\Sigma X,Y)=\operatorname {Map} (X,\Omega Y).$

These functors are used to construct fiber sequences and cofiber sequences. Namely, if $f:X\to Y$ is a map, the fiber sequence generated by $f$ is the exact sequence

$\cdots \to \Omega ^{2}Ff\to \Omega ^{2}X\to \Omega ^{2}Y\to \Omega Ff\to \Omega X\to \Omega Y\to Ff\to X\to Y$

where $Ff$ is the homotopy fiber of $f$; i.e., a fiber obtained after replacing $f$ by a (based) fibration. The cofibration sequence generated by $f$ is $X\to Y\to Cf\to \Sigma X\to \cdots ,$ where $Cf$ is the homotooy cofiber of $f$ constructed like a homotopy fiber (use a quotient instead of a fiber.)

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,

$[X,BG]=$ {principal G-bundle on X} / ~ $,\,\,[f]\mapsto [f^{*}EG]$

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

Main articles: Spectrum (algebraic topology) and Generalized cohomology

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 $\mathbb {Z}$),

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

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 K-theory is an example of a generalized cohomology theory.

A basic example of a spectrum is a sphere spectrum: $S^{0}\to S^{1}\to S^{2}\to \cdots$

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

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

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

Abstract homotopy theory is an axiomatic approach to homotopy theory. Such axiomatization is useful for non-traditional applications of homotopy theory. One approach to axiomatization is by Quillen's model categories. A model category is a category with a choice of three classes of maps called weak equivalences, cofibrations and fibrations, subject to the axioms that are reminiscent of facts in algebraic topology. For example, the category of (reasonable) topological spaces has a structure of a model category where a weak equivalence is a weak homotopy equivalence, a cofibration a certain retract and a fibration a Serre fibration. Another example is the category of non-negatively graded chain complexes over a fixed base ring.

Concepts

• fiber sequence
• cofiber sequence

Simplicial homotopy theory

• Simplicial homotopy

See also: Algebraic homotopy

See also

• Highly structured ring spectrum
• Homotopy type theory
• Pursuing Stacks
• Shape theory
• Moduli stack of formal group laws

References

• May, J. A Concise Course in Algebraic Topology
• George William Whitehead (1978). Elements of homotopy theory. Graduate Texts in Mathematics. Vol. 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.
• https://ncatlab.org/nlab/show/homotopical+algebra
• Homotopy Theories and Model Categories by W.G. Dwyer and J. Spalinski in Handbook of Algebraic Topology edited by I.M. James
• Hatcher, Allen. "Algebraic topology".
• Edwin Spanier, Algebraic topology

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
• D. Quillen, Homotopical algebra, Lectures Notes in Math. vol. 43, Springer Verlag, 1967.
• https://ncatlab.org/nlab/show/homotopy+theory

External links

"homotopy theory". ncatlab.org.