In

_{#} is a _{*} = ''g''_{*}. From this follows that if ''f'' is a homotopy equivalence, then ''f''_{*} is an isomorphism.
Let ''F'' : ''X'' × ^{''n''} → ''X'' of ''C_{n}''(''X'') to the "prism" ''P''(σ): Δ^{''n''} × ''I'' → ''Y''. The boundary of ''P''(σ) can be expressed as
:$\backslash partial\; P(\backslash sigma)\; =\; f\_(\backslash sigma)\; -\; g\_(\backslash sigma)\; -\; P(\backslash partial\; \backslash sigma).$
So if ''α'' in ''C_{n}''(''X'') is an ''n''-cycle, then ''f''_{#}(''α'' ) and ''g''_{#}(''α'') differ by a boundary:
:$f\_\; (\backslash alpha)\; -\; g\_(\backslash alpha)\; =\; \backslash partial\; P(\backslash alpha),$
i.e. they are homologous. This proves the claim.

^{''n''}, complex projective spaces, CP^{''n''}, a point, spheres ''S''^{n}($n\backslash ge\; 1$), and a 3-torus ''T''^{3} with integer coefficients.

_{''n''}(''X''; ''R'') should not be confused with the nearly identical notation ''H''_{''n''}(''X'', ''A''), which denotes the relative homology (below).
The

_{''n''}(''X'', ''A'') is understood to be the homology of the quotient of the chain complexes, that is,
:$H\_n(X,A)=H\_n(C\_\backslash bullet(X)/C\_\backslash bullet(A))$
where the quotient of chain complexes is given by the short exact sequence
:$0\backslash to\; C\_\backslash bullet(A)\; \backslash to\; C\_\backslash bullet(X)\; \backslash to\; C\_\backslash bullet(X)/C\_\backslash bullet(A)\; \backslash to\; 0.$

chain complex
In mathematics, a chain complex is an algebraic structure that consists of a sequence of abelian groups (or module (mathematics), modules) and a sequence of group homomorphism, homomorphisms between consecutive groups such that the image (mathemati ...

(i.e. applying the functor Hom(-, ''R''), ''R'' being any ring) we obtain a

''Algebraic topology.''

Cambridge University Press, and * J.P. May, ''A Concise Course in Algebraic Topology'', Chicago University Press * Joseph J. Rotman, ''An Introduction to Algebraic Topology'', Springer-Verlag, {{isbn, 0-387-96678-1 Homology theory

algebraic topology
Algebraic topology is a branch of mathematics that uses tools from abstract algebra to study topological spaces. The basic goal is to find algebraic invariant (mathematics), invariants that classification theorem, classify topological spaces up t ...

, singular homology refers to the study of a certain set of algebraic invariant
Invariant theory is a branch of abstract algebra dealing with actions of groups on algebraic varieties, such as vector spaces, from the point of view of their effect on functions. Classically, the theory dealt with the question of explicit descrip ...

s of a topological space
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called points ...

''X'', the so-called homology groups $H\_n(X).$ Intuitively, singular homology counts, for each dimension ''n'', the ''n''-dimensional holes of a space. Singular homology is a particular example of a homology theory
In mathematics, homology is a general way of associating a sequence of algebraic objects, such as abelian groups or modules, with other mathematical objects such as topological spaces. Homology groups were originally defined in algebraic topolog ...

, which has now grown to be a rather broad collection of theories. Of the various theories, it is perhaps one of the simpler ones to understand, being built on fairly concrete constructions (see also the related theory simplicial homology In algebraic topology, simplicial homology is the sequence of homology groups of a simplicial complex. It formalizes the idea of the number of holes of a given dimension in the complex. This generalizes the number of connected components (the case ...

).
In brief, singular homology is constructed by taking maps of the standard ''n''-simplex to a topological space, and composing them into formal sums, called singular chains. The boundary operation – mapping each ''n''-dimensional simplex to its (''n''−1)-dimensional boundary
Boundary or Boundaries may refer to:
* Border, in political geography
Entertainment
*Boundaries (2016 film), ''Boundaries'' (2016 film), a 2016 Canadian film
*Boundaries (2018 film), ''Boundaries'' (2018 film), a 2018 American-Canadian road trip ...

– induces the singular chain complex
In mathematics, a chain complex is an algebraic structure that consists of a sequence of abelian groups (or module (mathematics), modules) and a sequence of group homomorphism, homomorphisms between consecutive groups such that the image (mathemati ...

. The singular homology is then the homology
Homology may refer to:
Sciences
Biology
*Homology (biology), any characteristic of biological organisms that is derived from a common ancestor
* Sequence homology, biological homology between DNA, RNA, or protein sequences
*Homologous chrom ...

of the chain complex. The resulting homology groups are the same for all homotopy equivalent
In topology, a branch of mathematics, two continuous functions from one topological space to another are called homotopic (from grc, ὁμός "same, similar" and "place") if one can be "continuously deformed" into the other, such a deforma ...

spaces, which is the reason for their study. These constructions can be applied to all topological spaces, and so singular homology is expressible as a functor
In mathematics, specifically category theory, a functor is a Map (mathematics), mapping between Category (mathematics), categories. Functors were first considered in algebraic topology, where algebraic objects (such as the fundamental group) ar ...

from the category of topological spaces In mathematics, the category of topological spaces, often denoted Top, is the category whose objects are topological spaces and whose morphisms are continuous maps. This is a category because the composition of two continuous maps is again contin ...

to the category of graded abelian group
In mathematics, an abelian group, also called a commutative group, is a group in which the result of applying the group operation to two group elements does not depend on the order in which they are written. That is, the group operation is commut ...

s.
Singular simplices

A singular ''n''-simplex in a topological space ''X'' is acontinuous function
In mathematics, a continuous function is a function such that a continuous variation (that is a change without jump) of the argument induces a continuous variation of the value of the function. This means that there are no abrupt changes in value ...

(also called a map) $\backslash sigma$ from the standard ''n''-simplex
In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given dimension. ...

$\backslash Delta^n$ to ''X'', written $\backslash sigma:\backslash Delta^n\backslash to\; X.$ This map need not be injective
In mathematics, an injective function (also known as injection, or one-to-one function) is a function that maps distinct elements of its domain to distinct elements; that is, implies . (Equivalently, implies in the equivalent contrapositiv ...

, and there can be non-equivalent singular simplices with the same image in ''X''.
The boundary of $\backslash sigma,$ denoted as $\backslash partial\_n\backslash sigma,$ is defined to be the formal sum In mathematics, a formal sum, formal series, or formal linear combination may be:
*In group theory, an element of a free abelian group, a sum of finitely many elements from a given basis set multiplied by integer coefficients.
*In linear algebra, an ...

of the singular (''n'' − 1)-simplices represented by the restriction of $\backslash sigma$ to the faces of the standard ''n''-simplex, with an alternating sign to take orientation into account. (A formal sum is an element of the free abelian group
In mathematics, a free abelian group is an abelian group with a basis. Being an abelian group means that it is a set with an addition operation that is associative, commutative, and invertible. A basis, also called an integral basis, is a subse ...

on the simplices. The basis for the group is the infinite set of all possible singular simplices. The group operation is "addition" and the sum of simplex ''a'' with simplex ''b'' is usually simply designated ''a'' + ''b'', but ''a'' + ''a'' = 2''a'' and so on. Every simplex ''a'' has a negative −''a''.) Thus, if we designate $\backslash sigma$ by its vertices
:$;\; href="/html/ALL/l/\_0,p\_1,\backslash ldots,p\_n.html"\; ;"title="\_0,p\_1,\backslash ldots,p\_n">\_0,p\_1,\backslash ldots,p\_n$formal sum In mathematics, a formal sum, formal series, or formal linear combination may be:
*In group theory, an element of a free abelian group, a sum of finitely many elements from a given basis set multiplied by integer coefficients.
*In linear algebra, an ...

of the faces of the simplex image designated in a specific way. (That is, a particular face has to be the restriction of $\backslash sigma$ to a face of $\backslash Delta^n$ which depends on the order that its vertices are listed.) Thus, for example, the boundary of $\backslash sigma=;\; href="/html/ALL/l/\_0,p\_1.html"\; ;"title="\_0,p\_1">\_0,p\_1$Singular chain complex

The usual construction of singular homology proceeds by defining formal sums of simplices, which may be understood to be elements of afree abelian group
In mathematics, a free abelian group is an abelian group with a basis. Being an abelian group means that it is a set with an addition operation that is associative, commutative, and invertible. A basis, also called an integral basis, is a subse ...

, and then showing that we can define a certain group, the homology group of the topological space, involving the boundary operator.
Consider first the set of all possible singular ''n''-simplices $\backslash sigma\_n(X)$ on a topological space ''X''. This set may be used as the basis of a free abelian group
In mathematics, a free abelian group is an abelian group with a basis. Being an abelian group means that it is a set with an addition operation that is associative, commutative, and invertible. A basis, also called an integral basis, is a subse ...

, so that each singular ''n''-simplex is a generator of the group. This set of generators is of course usually infinite, frequently uncountable
In mathematics, an uncountable set (or uncountably infinite set) is an infinite set that contains too many elements to be countable. The uncountability of a set is closely related to its cardinal number: a set is uncountable if its cardinal num ...

, as there are many ways of mapping a simplex into a typical topological space. The free abelian group generated by this basis is commonly denoted as $C\_n(X)$. Elements of $C\_n(X)$ are called singular ''n''-chains; they are formal sums of singular simplices with integer coefficients.
The boundary
Boundary or Boundaries may refer to:
* Border, in political geography
Entertainment
*Boundaries (2016 film), ''Boundaries'' (2016 film), a 2016 Canadian film
*Boundaries (2018 film), ''Boundaries'' (2018 film), a 2018 American-Canadian road trip ...

$\backslash partial$ is readily extended to act on singular ''n''-chains. The extension, called the boundary operator
In mathematics, a chain complex is an algebraic structure that consists of a sequence of abelian groups (or modules) and a sequence of homomorphisms between consecutive groups such that the image of each homomorphism is included in the kernel of t ...

, written as
:$\backslash partial\_n:C\_n\backslash to\; C\_,$
is a homomorphism
In algebra, a homomorphism is a structure-preserving map between two algebraic structures of the same type (such as two groups, two rings, or two vector spaces). The word ''homomorphism'' comes from the Ancient Greek language: () meaning "same" ...

of groups. The boundary operator, together with the $C\_n$, form a chain complex
In mathematics, a chain complex is an algebraic structure that consists of a sequence of abelian groups (or module (mathematics), modules) and a sequence of group homomorphism, homomorphisms between consecutive groups such that the image (mathemati ...

of abelian groups, called the singular complex. It is often denoted as $(C\_\backslash bullet(X),\backslash partial\_\backslash bullet)$ or more simply $C\_\backslash bullet(X)$.
The kernel of the boundary operator is $Z\_n(X)=\backslash ker\; (\backslash partial\_)$, and is called the group of singular ''n''-cycles. The image of the boundary operator is $B\_n(X)=\backslash operatorname\; (\backslash partial\_)$, and is called the group of singular ''n''-boundaries.
It can also be shown that $\backslash partial\_n\backslash circ\; \backslash partial\_=0$, implying $B\_n(X)\; \backslash subseteq\; Z\_n(X)$. The $n$-th homology group of $X$ is then defined as the factor group
Factor, a Latin word meaning "who/which acts", may refer to:
Commerce
* Factor (agent), a person who acts for, notably a mercantile and colonial agent
* Factor (Scotland), a person or firm managing a Scottish estate
* Factors of production, suc ...

:$H\_(X)\; =\; Z\_n(X)\; /\; B\_n(X).$
The elements of $H\_n(X)$ are called homology classes.
Homotopy invariance

If ''X'' and ''Y'' are two topological spaces with the samehomotopy type
In topology, a branch of mathematics, two continuous functions from one topological space to another are called homotopic (from grc, ὁμός "same, similar" and "place") if one can be "continuously deformed" into the other, such a deforma ...

(i.e. are homotopy equivalent
In topology, a branch of mathematics, two continuous functions from one topological space to another are called homotopic (from grc, ὁμός "same, similar" and "place") if one can be "continuously deformed" into the other, such a deforma ...

), then
:$H\_n(X)\; \backslash cong\; H\_n(Y)\backslash ,$
for all ''n'' ≥ 0. This means homology groups are homotopy invariants, and therefore topological invariant
In topology and related areas of mathematics, a topological property or topological invariant is a property of a topological space that is invariant under homeomorphisms. Alternatively, a topological property is a proper class of topological space ...

s.
In particular, if ''X'' is a connected contractible space
In mathematics, a topological space ''X'' is contractible if the identity map on ''X'' is null-homotopic, i.e. if it is homotopic to some constant map. Intuitively, a contractible space is one that can be continuously shrunk to a point within that ...

, then all its homology groups are 0, except $H\_0(X)\; \backslash cong\; \backslash mathbb$.
A proof for the homotopy invariance of singular homology groups can be sketched as follows. A continuous map ''f'': ''X'' → ''Y'' induces a homomorphism
:$f\_\; :\; C\_n(X)\; \backslash rightarrow\; C\_n(Y).$
It can be verified immediately that
:$\backslash partial\; f\_\; =\; f\_\; \backslash partial,$
i.e. ''f''chain map
A chain is a serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression but linear, rigid, and load-bearing in tension. A c ...

, which descends to homomorphisms on homology
:$f\_*\; :\; H\_n(X)\; \backslash rightarrow\; H\_n(Y).$
We now show that if ''f'' and ''g'' are homotopically equivalent, then ''f'', 1
The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline (t ...

→ ''Y'' be a homotopy that takes ''f'' to ''g''. On the level of chains, define a homomorphism
:$P\; :\; C\_n(X)\; \backslash rightarrow\; C\_(Y)$
that, geometrically speaking, takes a basis element σ: ΔHomology groups of common spaces

The table below shows the k-th homology groups $H\_k(X)$ of n-dimensional real projective spaces RPFunctoriality

The construction above can be defined for any topological space, and is preserved by the action of continuous maps. This generality implies that singular homology theory can be recast in the language ofcategory 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, cate ...

. In particular, the homology group can be understood to be a functor
In mathematics, specifically category theory, a functor is a Map (mathematics), mapping between Category (mathematics), categories. Functors were first considered in algebraic topology, where algebraic objects (such as the fundamental group) ar ...

from the category of topological spaces In mathematics, the category of topological spaces, often denoted Top, is the category whose objects are topological spaces and whose morphisms are continuous maps. This is a category because the composition of two continuous maps is again contin ...

Top to the category of abelian groups In mathematics, the category Ab has the abelian groups as objects and group homomorphisms as morphisms. This is the prototype of an abelian category: indeed, every small abelian category can be embedded in Ab.
Properties
The zero object of Ab is ...

Ab.
Consider first that $X\backslash mapsto\; C\_n(X)$ is a map from topological spaces to free abelian groups. This suggests that $C\_n(X)$ might be taken to be a functor, provided one can understand its action on the morphism
In mathematics, particularly in category theory, a morphism is a structure-preserving map from one mathematical structure to another one of the same type. The notion of morphism recurs in much of contemporary mathematics. In set theory, morphisms a ...

s of Top. Now, the morphisms of Top are continuous functions, so if $f:X\backslash to\; Y$ is a continuous map of topological spaces, it can be extended to a homomorphism of groups
:$f\_*:C\_n(X)\backslash to\; C\_n(Y)\backslash ,$
by defining
:$f\_*\backslash left(\backslash sum\_i\; a\_i\backslash sigma\_i\backslash right)=\backslash sum\_i\; a\_i\; (f\backslash circ\; \backslash sigma\_i)$
where $\backslash sigma\_i:\backslash Delta^n\backslash to\; X$ is a singular simplex, and $\backslash sum\_i\; a\_i\backslash sigma\_i\backslash ,$ is a singular ''n''-chain, that is, an element of $C\_n(X)$. This shows that $C\_n$ is a functor
:$C\_n:\backslash mathbf\; \backslash to\; \backslash mathbf$
from the category of topological spaces In mathematics, the category of topological spaces, often denoted Top, is the category whose objects are topological spaces and whose morphisms are continuous maps. This is a category because the composition of two continuous maps is again contin ...

to the category of abelian groups In mathematics, the category Ab has the abelian groups as objects and group homomorphisms as morphisms. This is the prototype of an abelian category: indeed, every small abelian category can be embedded in Ab.
Properties
The zero object of Ab is ...

.
The boundary operator commutes with continuous maps, so that $\backslash partial\_n\; f\_*=f\_*\backslash partial\_n$. This allows the entire chain complex to be treated as a functor. In particular, this shows that the map $X\backslash mapsto\; H\_n\; (X)$ is a functor
In mathematics, specifically category theory, a functor is a Map (mathematics), mapping between Category (mathematics), categories. Functors were first considered in algebraic topology, where algebraic objects (such as the fundamental group) ar ...

:$H\_n:\backslash mathbf\backslash to\backslash mathbf$
from the category of topological spaces to the category of abelian groups. By the homotopy axiom, one has that $H\_n$ is also a functor, called the homology functor, acting on hTop, the quotient homotopy category In mathematics, the homotopy category is a category built from the category of topological spaces which in a sense identifies two spaces that have the same shape. The phrase is in fact used for two different (but related) categories, as discussed be ...

:
:$H\_n:\backslash mathbf\backslash to\backslash mathbf.$
This distinguishes singular homology from other homology theories, wherein $H\_n$ is still a functor, but is not necessarily defined on all of Top. In some sense, singular homology is the "largest" homology theory, in that every homology theory on a subcategory of Top agrees with singular homology on that subcategory. On the other hand, the singular homology does not have the cleanest categorical properties; such a cleanup motivates the development of other homology theories such as cellular homology In mathematics, cellular homology in algebraic topology is a homology theory for the category of CW-complexes. It agrees with singular homology, and can provide an effective means of computing homology modules.
Definition
If X is a CW-complex ...

.
More generally, the homology functor is defined axiomatically, as a functor on an abelian category
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 ab ...

, or, alternately, as a functor on chain complex
In mathematics, a chain complex is an algebraic structure that consists of a sequence of abelian groups (or module (mathematics), modules) and a sequence of group homomorphism, homomorphisms between consecutive groups such that the image (mathemati ...

es, satisfying axioms that require a boundary morphism that turns short exact sequence
An exact sequence is a sequence of morphisms between objects (for example, groups, rings, modules, and, more generally, objects of an abelian category) such that the image of one morphism equals the kernel of the next.
Definition
In the context o ...

s into long exact sequence
An exact sequence is a sequence of morphisms between objects (for example, groups, rings, modules, and, more generally, objects of an abelian category) such that the image of one morphism equals the kernel of the next.
Definition
In the context ...

s. In the case of singular homology, the homology functor may be factored into two pieces, a topological piece and an algebraic piece. The topological piece is given by
:$C\_\backslash bullet:\backslash mathbf\backslash to\backslash mathbf$
which maps topological spaces as $X\backslash mapsto\; (C\_\backslash bullet(X),\backslash partial\_\backslash bullet)$ and continuous functions as $f\backslash mapsto\; f\_*$. Here, then, $C\_\backslash bullet$ is understood to be the singular chain functor, which maps topological spaces to the category of chain complexes Comp (or Kom). The category of chain complexes has chain complexes as its object
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 ...

s, and chain map
A chain is a serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression but linear, rigid, and load-bearing in tension. A c ...

s as its morphism
In mathematics, particularly in category theory, a morphism is a structure-preserving map from one mathematical structure to another one of the same type. The notion of morphism recurs in much of contemporary mathematics. In set theory, morphisms a ...

s.
The second, algebraic part is the homology functor
:$H\_n:\backslash mathbf\backslash to\backslash mathbf$
which maps
:$C\_\backslash bullet\backslash mapsto\; H\_n(C\_\backslash bullet)=Z\_n(C\_\backslash bullet)/B\_n(C\_\backslash bullet)$
and takes chain maps to maps of abelian groups. It is this homology functor that may be defined axiomatically, so that it stands on its own as a functor on the category of chain complexes.
Homotopy maps re-enter the picture by defining homotopically equivalent chain maps. Thus, one may define the quotient category
In mathematics, a quotient category is a category obtained from another one by identifying sets of morphisms. Formally, it is a quotient object in the category of (locally small) categories, analogous to a quotient group or quotient space, but in ...

hComp or K, the homotopy category of chain complexes In homological algebra in mathematics, the homotopy category ''K(A)'' of chain complexes in an additive category ''A'' is a framework for working with chain homotopies and homotopy equivalences. It lies intermediate between the category of chain c ...

.
Coefficients in ''R''

Given any unitalring
Ring may refer to:
* Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry
* To make a sound with a bell, and the sound made by a bell
:(hence) to initiate a telephone connection
Arts, entertainment and media Film and ...

''R'', the set of singular ''n''-simplices on a topological space can be taken to be the generators of a free ''R''-module. That is, rather than performing the above constructions from the starting point of free abelian groups, one instead uses free ''R''-modules in their place. All of the constructions go through with little or no change. The result of this is
:$H\_n(X;\; R)\backslash $
which is now an ''R''-module. Of course, it is usually ''not'' a free module. The usual homology group is regained by noting that
:$H\_n(X;\backslash mathbb)=H\_n(X)$
when one takes the ring to be the ring of integers. The notation ''H''universal coefficient theorem
In algebraic topology, universal coefficient theorems establish relationships between homology groups (or cohomology groups) with different coefficients. For instance, for every topological space , its ''integral homology groups'':
:
completely ...

provides a mechanism to calculate the homology with ''R'' coefficients in terms of homology with usual integer coefficients using the short exact sequence
:$0\backslash to\; H\_n(X;\; \backslash mathbb)\; \backslash otimes\; R\; \backslash to\; H\_n(X;\; R)\; \backslash to\; Tor(H\_(X;\; \backslash mathbb),\; R)\; \backslash to\; 0.$
where ''Tor'' is the Tor functor
In mathematics, the Tor functors are the derived functors of the tensor product of modules over a ring. Along with the Ext functor, Tor is one of the central concepts of homological algebra, in which ideas from algebraic topology are used to con ...

.
Of note, if ''R'' is torsion-free, then ''Tor(G, R)'' = 0 for any G, so the above short exact sequence reduces to an isomorphism between $H\_n(X;\; \backslash mathbb)\; \backslash otimes\; R$ and $H\_n(X;\; R).$
Relative homology

For a subspace $A\backslash subset\; X$, therelative homology In algebraic topology, a branch of mathematics, the (singular) homology of a topological space relative to a subspace is a construction in singular homology, for pairs of spaces. The relative homology is useful and important in several ways. Intui ...

''H''Reduced homology

Thereduced homology
In mathematics, reduced homology is a minor modification made to homology theory in algebraic topology, motivated by the intuition that all of the homology groups of a single point should be equal to zero. This modification allows more concise stat ...

of a space ''X'', annotated as $\backslash tilde\_n(X)$ is a minor modification to the usual homology which simplifies expressions of some relationships and fulfils the intuiton that all homology groups of a point should be zero.
For the usual homology defined on a chain complex:
:$\backslash dotsb\backslash oversetC\_n\; \backslash oversetC\_\; \backslash overset\; \backslash dotsb\; \backslash overset\; C\_1\; \backslash overset\; C\_0\backslash overset\; 0$
To define the reduced homology, we augment the chain complex with an additional $\backslash mathbb$ between $C\_0$ and zero:
$\backslash dotsb\backslash oversetC\_n\; \backslash oversetC\_\; \backslash overset\; \backslash dotsb\; \backslash overset\; C\_1\; \backslash overset\; C\_0\backslash overset\; \backslash mathbb\; \backslash to\; 0$
where $\backslash epsilon\; \backslash left(\; \backslash sum\_i\; n\_i\; \backslash sigma\_i\; \backslash right)\; =\; \backslash sum\_i\; n\_i$. This can be justified by interpreting the empty set as "(-1)-simplex", which means that $C\_\; \backslash simeq\; \backslash Z$.
The ''reduced'' homology groups are now defined by $\backslash tilde\_n(X)\; =\; \backslash ker(\backslash partial\_n)\; /\; \backslash mathrm(\backslash partial\_)$ for positive ''n'' and $\backslash tilde\_0(X)\; =\; \backslash ker(\backslash epsilon)\; /\; \backslash mathrm(\backslash partial\_1)$. Hatcher, 110
For n > 0, $H\_n(X)\; =\; \backslash tilde\_n(X)$, while for n = 0, $H\_0(X)\; =\; \backslash tilde\_0(X)\; \backslash oplus\; \backslash mathbb.$
Cohomology

By dualizing the homologycochain complex
In mathematics, a chain complex is an algebraic structure that consists of a sequence of abelian groups (or modules) and a sequence of homomorphisms between consecutive groups such that the image of each homomorphism is included in the kernel of th ...

with coboundary map $\backslash delta$. The cohomology groups of ''X'' are defined as the homology groups of this complex; in a quip, "cohomology is the homology of the co he dual complex.
The cohomology groups have a richer, or at least more familiar, algebraic structure than the homology groups. Firstly, they form a differential graded algebra
In mathematics, in particular abstract algebra and topology, a differential graded algebra is a graded associative algebra with an added chain complex structure that respects the algebra structure.
__TOC__
Definition
A differential graded alg ...

as follows:
* the graded set of groups form a graded ''R''-module
Module, modular and modularity may refer to the concept of modularity. They may also refer to:
Computing and engineering
* Modular design, the engineering discipline of designing complex devices using separately designed sub-components
* Mo ...

;
* this can be given the structure of a graded ''R''-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 a ...

using the cup product In mathematics, specifically in algebraic topology, the cup product is a method of adjoining two cocycles of degree ''p'' and ''q'' to form a composite cocycle of degree ''p'' + ''q''. This defines an associative (and distributive) graded commutat ...

;
* the Bockstein homomorphism ''β'' gives a differential.
There are additional cohomology operation In mathematics, the cohomology operation concept became central to algebraic topology, particularly homotopy theory, from the 1950s onwards, in the shape of the simple definition that if ''F'' is a functor defining a cohomology theory, then a coho ...

s, and the cohomology algebra has addition structure mod ''p'' (as before, the mod ''p'' cohomology is the cohomology of the mod ''p'' cochain complex, not the mod ''p'' reduction of the cohomology), notably the Steenrod algebra structure.
Betti homology and cohomology

Since the number of homology theories has become large (see :Homology theory), the terms ''Betti homology'' and ''Betti cohomology'' are sometimes applied (particularly by authors writing onalgebraic geometry
Algebraic geometry is a branch of mathematics, classically studying zeros of multivariate polynomials. Modern algebraic geometry is based on the use of abstract algebraic techniques, mainly from commutative algebra, for solving geometrical ...

) to the singular theory, as giving rise to the Betti number
In algebraic topology, the Betti numbers are used to distinguish topological spaces based on the connectivity of ''n''-dimensional simplicial complexes. For the most reasonable finite-dimensional spaces (such as compact manifolds, finite simplicia ...

s of the most familiar spaces such as simplicial complex
In mathematics, a simplicial complex is a set composed of points, line segments, triangles, and their ''n''-dimensional counterparts (see illustration). Simplicial complexes should not be confused with the more abstract notion of a simplicial set ...

es and closed manifold
In mathematics, a closed manifold is a manifold without boundary that is compact.
In comparison, an open manifold is a manifold without boundary that has only ''non-compact'' components.
Examples
The only connected one-dimensional example ...

s.
Extraordinary homology

If one defines a homology theory axiomatically (via theEilenberg–Steenrod axioms
In mathematics, specifically in algebraic topology, the Eilenberg–Steenrod axioms are properties that homology theories of topological spaces have in common. The quintessential example of a homology theory satisfying the axioms is singular homo ...

), and then relaxes one of the axioms (the ''dimension axiom''), one obtains a generalized theory, called an extraordinary homology theory
In mathematics, specifically in homology theory and algebraic topology, cohomology is a general term for a sequence of abelian groups, usually one associated with a topological space, often defined from a cochain complex. Cohomology can be viewe ...

. These originally arose in the form of extraordinary cohomology theories
In mathematics, specifically in homology theory and algebraic topology, cohomology is a general term for a sequence of abelian groups, usually one associated with a topological space, often defined from a cochain complex. Cohomology can be viewe ...

, namely K-theory
In mathematics, K-theory is, roughly speaking, the study of a ring generated by vector bundles over a topological space or scheme. In algebraic topology, it is a cohomology theory known as topological K-theory. In algebra and algebraic geometry, ...

and cobordism theory
In mathematics, cobordism is a fundamental equivalence relation on the class of compact manifolds of the same dimension, set up using the concept of the boundary (French '' bord'', giving ''cobordism'') of a manifold. Two manifolds of the same ...

. In this context, singular homology is referred to as ordinary homology.
See also

*Derived category
In mathematics, the derived category ''D''(''A'') of an abelian category ''A'' is a construction of homological algebra introduced to refine and in a certain sense to simplify the theory of derived functors defined on ''A''. The construction proce ...

* Excision theorem
In algebraic topology, a branch of mathematics, the excision theorem is a theorem about relative homology and one of the Eilenberg–Steenrod axioms. Given a topological space X and subspaces A and U such that U is also a subspace of A, the theorem ...

* Hurewicz theorem
In mathematics, the Hurewicz theorem is a basic result of algebraic topology, connecting homotopy theory with homology theory via a map known as the Hurewicz homomorphism. The theorem is named after Witold Hurewicz, and generalizes earlier results ...

* Simplicial homology In algebraic topology, simplicial homology is the sequence of homology groups of a simplicial complex. It formalizes the idea of the number of holes of a given dimension in the complex. This generalizes the number of connected components (the case ...

* Cellular homology In mathematics, cellular homology in algebraic topology is a homology theory for the category of CW-complexes. It agrees with singular homology, and can provide an effective means of computing homology modules.
Definition
If X is a CW-complex ...

References

* Allen Hatcher''Algebraic topology.''

Cambridge University Press, and * J.P. May, ''A Concise Course in Algebraic Topology'', Chicago University Press * Joseph J. Rotman, ''An Introduction to Algebraic Topology'', Springer-Verlag, {{isbn, 0-387-96678-1 Homology theory