In mathematics, homology^{[1]} 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 topology. Similar constructions are available in a wide variety of other contexts, such as abstract algebra, groups, Lie algebras, Galois theory, and algebraic geometry.
The original motivation for defining homology groups was the observation that two shapes can be distinguished by examining their holes. For instance, a circle is not a disk because the circle has a hole through it while the disk is solid, and the ordinary sphere is not a circle because the sphere encloses a twodimensional hole while the circle encloses a onedimensional hole. However, because a hole is "not there", it is not immediately obvious how to define a hole or how to distinguish different kinds of holes. Homology was originally a rigorous mathematical method for defining and categorizing holes in a manifold. Loosely speaking, a cycle is a closed submanifold, a boundary is a cycle which is also the boundary of a submanifold, and a homology class (which represents a hole) is an equivalence class of cycles modulo boundaries. A homology class is thus represented by a cycle which is not the boundary of any submanifold: the cycle represents a hole, namely a hypothetical manifold whose boundary would be that cycle, but which is "not there".
There are many different homology theories. A particular type of mathematical object, such as a topological space or a group, may have one or more associated homology theories. When the underlying object has a geometric interpretation as topological spaces do, the nth homology group represents behavior in dimension n. Most homology groups or modules may be formulated as derived functors on appropriate abelian categories, measuring the failure of a functor to be exact. From this abstract perspective, homology groups are determined by objects of a derived category.
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Transcription
Background
Origins
Homology theory can be said to start with the Euler polyhedron formula, or Euler characteristic.^{[2]} This was followed by Riemann's definition of genus and nfold connectedness numerical invariants in 1857 and Betti's proof in 1871 of the independence of "homology numbers" from the choice of basis.^{[3]}
Homology itself was developed as a way to analyse and classify manifolds according to their cycles – closed loops (or more generally submanifolds) that can be drawn on a given n dimensional manifold but not continuously deformed into each other.^{[4]} These cycles are also sometimes thought of as cuts which can be glued back together, or as zippers which can be fastened and unfastened. Cycles are classified by dimension. For example, a line drawn on a surface represents a 1cycle, a closed loop or (1manifold), while a surface cut through a threedimensional manifold is a 2cycle.
Surfaces
On the ordinary sphere , the cycle b in the diagram can be shrunk to the pole, and even the equatorial great circle a can be shrunk in the same way. The Jordan curve theorem shows that any arbitrary cycle such as c can be similarly shrunk to a point. All cycles on the sphere can therefore be continuously transformed into each other and belong to the same homology class. They are said to be homologous to zero. Cutting a manifold along a cycle homologous to zero separates the manifold into two or more components. For example, cutting the sphere along a produces two hemispheres.
This is not generally true of cycles on other surfaces. The torus has cycles which cannot be continuously deformed into each other, for example in the diagram none of the cycles a, b or c can be deformed into one another. In particular, cycles a and b cannot be shrunk to a point whereas cycle c can, thus making it homologous to zero.
If the torus surface is cut along both a and b, it can be opened out and flattened into a rectangle or, more conveniently, a square. One opposite pair of sides represents the cut along a, and the other opposite pair represents the cut along b.
The edges of the square may then be glued back together in different ways. The square can be twisted to allow edges to meet in the opposite direction, as shown by the arrows in the diagram. Up to symmetry, there are four distinct ways of gluing the sides, each creating a different surface:
is the Klein bottle, which is a torus with a twist in it (The twist can be seen in the square diagram as the reversal of the bottom arrow). It is a theorem that the reglued surface must selfintersect (when immersed in Euclidean 3space). Like the torus, cycles a and b cannot be shrunk while c can be. But unlike the torus, following b forwards right round and back reverses left and right, because b happens to cross over the twist given to one join. If an equidistant cut on one side of b is made, it returns on the other side and goes round the surface a second time before returning to its starting point, cutting out a twisted Möbius strip. Because local left and right can be arbitrarily reoriented in this way, the surface as a whole is said to be nonorientable.
The projective plane has both joins twisted. The uncut form, generally represented as the Boy surface, is visually complex, so a hemispherical embedding is shown in the diagram, in which antipodal points around the rim such as A and A′ are identified as the same point. Again, a and b are nonshrinkable while c is. But this time, both a and b reverse left and right.
Cycles can be joined or added together, as a and b on the torus were when it was cut open and flattened down. In the Klein bottle diagram, a goes round one way and −a goes round the opposite way. If a is thought of as a cut, then −a can be thought of as a gluing operation. Making a cut and then regluing it does not change the surface, so a + (−a) = 0.
But now consider two acycles. Since the Klein bottle is nonorientable, you can transport one of them all the way round the bottle (along the bcycle), and it will come back as −a. This is because the Klein bottle is made from a cylinder, whose acycle ends are glued together with opposite orientations. Hence 2a = a + a = a + (−a) = 0. This phenomenon is called torsion. Similarly, in the projective plane, following the unshrinkable cycle b round twice remarkably creates a trivial cycle which can be shrunk to a point; that is, b + b = 0. Because b must be followed around twice to achieve a zero cycle, the surface is said to have a torsion coefficient of 2. However, following a bcycle around twice in the Klein bottle gives simply b + b = 2b, since this cycle lives in a torsionfree homology class. This corresponds to the fact that in the fundamental polygon of the Klein bottle, only one pair of sides is glued with a twist, whereas in the projective plane both sides are twisted.
A square is a contractible topological space, which implies that it has trivial homology. Consequently, additional cuts disconnect it. The square is not the only shape in the plane that can be glued into a surface. Gluing opposite sides of an octagon, for example, produces a surface with two holes. In fact, all closed surfaces can be produced by gluing the sides of some polygon and all evensided polygons (2ngons) can be glued to make different manifolds. Conversely, a closed surface with n nonzero classes can be cut into a 2ngon. Variations are also possible, for example a hexagon may also be glued to form a torus.^{[5]}
The first recognisable theory of homology was published by Henri Poincaré in his seminal paper "Analysis situs", J. Ecole polytech. (2) 1. 1–121 (1895). The paper introduced homology classes and relations. The possible configurations of orientable cycles are classified by the Betti numbers of the manifold (Betti numbers are a refinement of the Euler characteristic). Classifying the nonorientable cycles requires additional information about torsion coefficients.^{[4]}
The complete classification of 1 and 2manifolds is given in the table.
Manifold  Euler no., χ 
Orientability  Betti numbers  Torsion coefficient (1dimensional)  

Symbol^{[5]}  Name  b_{0}  b_{1}  b_{2}  
Circle (1manifold)  0  Orientable  1  1  —  —  
Sphere  2  Orientable  1  0  1  None  
Torus  0  Orientable  1  2  1  None  
Projective plane  1  Nonorientable  1  0  0  2  
Klein bottle  0  Nonorientable  1  1  0  2  
2holed torus  −2  Orientable  1  4  1  None  
gholed torus (g is the genus)  2 − 2g  Orientable  1  2g  1  None  
Sphere with c crosscaps  2 − c  Nonorientable  1  c − 1  0  2  
2Manifold with g holes and c crosscaps (c > 0)  2 − (2g + c)  Nonorientable  1  (2g + c) − 1  0  2 
 Notes
 For a nonorientable surface, a hole is equivalent to two crosscaps.
 Any 2manifold is the connected sum of g tori and c projective planes. For the sphere , g = c = 0.
Generalization
A manifold with boundary or open manifold is topologically distinct from a closed manifold and can be created by making a cut in any suitable closed manifold. For example the disk or 2ball is bounded by a circle .^{[citation needed]} It may be created by cutting a trivial cycle in any 2manifold and keeping the piece removed, by piercing the sphere and stretching the puncture wide, or by cutting the projective plane. It can also be seen as fillingin the circle in the plane.
When two cycles can be continuously deformed into each other, then cutting along one produces the same shape as cutting along the other, up to some bending and stretching. In this case the two cycles are said to be homologous or to lie in the same homology class. Additionally, if one cycle can be continuously deformed into a combination of other cycles, then cutting along the initial cycle is the same as cutting along the combination of other cycles. For example, cutting along a figure 8 is equivalent to cutting along its two lobes. In this case, the figure 8 is said to be homologous to the sum of its lobes.
Two open manifolds with similar boundaries (up to some bending and stretching) may be glued together to form a new manifold which is their connected sum.
This geometric analysis of manifolds is not rigorous. In a search for increased rigour, Poincaré went on to develop the simplicial homology of a triangulated manifold and to create what is now called a chain complex.^{[7]}^{[8]} These chain complexes (since greatly generalized) form the basis for most modern treatments of homology.
In such treatments a cycle need not be continuous: a 0cycle is a set of points, and cutting along this cycle corresponds to puncturing the manifold. A 1cycle corresponds to a set of closed loops (an image of the 1manifold ). On a surface, cutting along a 1cycle yields either disconnected pieces or a simpler shape. A 2cycle corresponds to a collection of embedded surfaces such as a sphere or a torus, and so on.
Emmy Noether and, independently, Leopold Vietoris and Walther Mayer further developed the theory of algebraic homology groups in the period 1925–28.^{[9]}^{[10]}^{[11]} The new combinatorial topology formally treated topological classes as abelian groups. Homology groups are finitely generated abelian groups, and homology classes are elements of these groups. The Betti numbers of the manifold are the rank of the free part of the homology group, and the nonorientable cycles are described by the torsion part.
The subsequent spread of homology groups brought a change of terminology and viewpoint from "combinatorial topology" to "algebraic topology".^{[12]} Algebraic homology remains the primary method of classifying manifolds.^{[13]}
Informal examples
The homology of a topological space X is a set of topological invariants of X represented by its homology groups
A onedimensional sphere is a circle. It has a single connected component and a onedimensionalboundary hole, but no higherdimensional holes. The corresponding homology groups are given as
A twodimensional sphere has a single connected component, no onedimensionalboundary holes, a twodimensionalboundary hole, and no higherdimensional holes. The corresponding homology groups are^{[15]}^{[16]}
In general for an ndimensional sphere the homology groups are
A twodimensional ball is a solid disc. It has a single pathconnected component, but in contrast to the circle, has no higherdimensional holes. The corresponding homology groups are all trivial except for . In general, for an ndimensional ball ^{[15]}
The torus is defined as a product of two circles . The torus has a single pathconnected component, two independent onedimensional holes (indicated by circles in red and blue) and one twodimensional hole as the interior of the torus. The corresponding homology groups are^{[17]}
The two independent 1dimensional holes form independent generators in a finitelygenerated abelian group, expressed as the product group
For the projective plane P, a simple computation shows (where is the cyclic group of order 2):^{[18]}
corresponds, as in the previous examples, to the fact that there is a single connected component. is a new phenomenon: intuitively, it corresponds to the fact that there is a single noncontractible "loop", but if we do the loop twice, it becomes contractible to zero. This phenomenon is called torsion.
Construction of homology groups
The following text describes a general algorithm for constructing the homology groups. It may be easier for the reader to look at some simple examples first: graph homology and simplicial homology.
The general construction begins with an object such as a topological space X, on which one first defines a chain complex C(X) encoding information about X. A chain complex is a sequence of abelian groups or modules . connected by homomorphisms which are called boundary operators.^{[17]} That is,
where 0 denotes the trivial group and for i < 0. It is also required that the composition of any two consecutive boundary operators be trivial. That is, for all n,
i.e., the constant map sending every element of to the group identity in
The statement that the boundary of a boundary is trivial is equivalent to the statement that , where denotes the image of the boundary operator and its kernel. Elements of are called boundaries and elements of are called cycles.
Since each chain group C_{n} is abelian all its subgroups are normal. Then because is a subgroup of C_{n}, is abelian, and since therefore is a normal subgroup of . Then one can create the quotient group
called the nth homology group of X. The elements of H_{n}(X) are called homology classes. Each homology class is an equivalence class over cycles and two cycles in the same homology class are said to be homologous.^{[19]}
A chain complex is said to be exact if the image of the (n+1)th map is always equal to the kernel of the nth map. The homology groups of X therefore measure "how far" the chain complex associated to X is from being exact.^{[20]}
The reduced homology groups of a chain complex C(X) are defined as homologies of the augmented chain complex^{[21]}
where the boundary operator is
for a combination of points which are the fixed generators of C_{0}. The reduced homology groups coincide with for The extra in the chain complex represents the unique map from the empty simplex to X.
Computing the cycle and boundary groups is usually rather difficult since they have a very large number of generators. On the other hand, there are tools which make the task easier.
The simplicial homology groups H_{n}(X) of a simplicial complex X are defined using the simplicial chain complex C(X), with C_{n}(X) the free abelian group generated by the nsimplices of X. See simplicial homology for details.
The singular homology groups H_{n}(X) are defined for any topological space X, and agree with the simplicial homology groups for a simplicial complex.
Cohomology groups are formally similar to homology groups: one starts with a cochain complex, which is the same as a chain complex but whose arrows, now denoted point in the direction of increasing n rather than decreasing n; then the groups of cocycles and of coboundaries follow from the same description. The nth cohomology group of X is then the quotient group
in analogy with the nth homology group.
Homology vs. homotopy
Homotopy groups are similar to homology groups in that they can represent "holes" in a topological space. There is a close connection between the first homotopy group and the first homology group : the latter is the abelianization of the former. Hence, it is said that "homology is a commutative alternative to homotopy".^{[22]}^{: 4:00 } The higher homotopy groups are abelian and are related to homology groups by the Hurewicz theorem, but can be vastly more complicated. For instance, the homotopy groups of spheres are poorly understood and are not known in general, in contrast to the straightforward description given above for the homology groups.
As an example, let X be the figure eight. Its first homotopy group is the group of directed loops starting and ending at a predetermined point (e.g. its center). It is equivalent to the free group of rank 2, which is not commutative: looping around the leftmost cycle and then around the rightmost cycle is different than looping around the rightmost cycle and then looping around the leftmost cycle. In contrast, its first homology group is the group of cuts made in a surface. This group is commutative, since (informally) cutting the leftmost cycle and then the rightmost cycle leads to the same result as cutting the rightmost cycle and then the leftmost cycle.
Types of homology
The different types of homology theory arise from functors mapping from various categories of mathematical objects to the category of chain complexes. In each case the composition of the functor from objects to chain complexes and the functor from chain complexes to homology groups defines the overall homology functor for the theory.^{[23]}
Simplicial homology
The motivating example comes from algebraic topology: the simplicial homology of a simplicial complex X. Here the chain group C_{n} is the free abelian group or module whose generators are the ndimensional oriented simplexes of X. The orientation is captured by ordering the complex's vertices and expressing an oriented simplex as an ntuple of its vertices listed in increasing order (i.e. in the complex's vertex ordering, where is the th vertex appearing in the tuple). The mapping from C_{n} to C_{n−1} is called the boundary mapping and sends the simplex
to the formal sum
which is considered 0 if This behavior on the generators induces a homomorphism on all of C_{n} as follows. Given an element , write it as the sum of generators where is the set of nsimplexes in X and the m_{i} are coefficients from the ring C_{n} is defined over (usually integers, unless otherwise specified). Then define
The dimension of the nth homology of X turns out to be the number of "holes" in X at dimension n. It may be computed by putting matrix representations of these boundary mappings in Smith normal form.
Singular homology
Using simplicial homology example as a model, one can define a singular homology for any topological space X. A chain complex for X is defined by taking C_{n} to be the free abelian group (or free module) whose generators are all continuous maps from ndimensional simplices into X. The homomorphisms ∂_{n} arise from the boundary maps of simplexes.
Group homology
In abstract algebra, one uses homology to define derived functors, for example the Tor functors. Here one starts with some covariant additive functor F and some module X. The chain complex for X is defined as follows: first find a free module and a surjective homomorphism Then one finds a free module and a surjective homomorphism Continuing in this fashion, a sequence of free modules and homomorphisms can be defined. By applying the functor F to this sequence, one obtains a chain complex; the homology of this complex depends only on F and X and is, by definition, the nth derived functor of F, applied to X.
A common use of group (co)homology is to classify the possible extension groups E which contain a given Gmodule M as a normal subgroup and have a given quotient group G, so that
Other homology theories
Homology functors
Chain complexes form a category: A morphism from the chain complex () to the chain complex () is a sequence of homomorphisms such that for all n. The nth homology H_{n} can be viewed as a covariant functor from the category of chain complexes to the category of abelian groups (or modules).
If the chain complex depends on the object X in a covariant manner (meaning that any morphism induces a morphism from the chain complex of X to the chain complex of Y), then the H_{n} are covariant functors from the category that X belongs to into the category of abelian groups (or modules).
The only difference between homology and cohomology is that in cohomology the chain complexes depend in a contravariant manner on X, and that therefore the homology groups (which are called cohomology groups in this context and denoted by H^{n}) form contravariant functors from the category that X belongs to into the category of abelian groups or modules.
Properties
If () is a chain complex such that all but finitely many A_{n} are zero, and the others are finitely generated abelian groups (or finitedimensional vector spaces), then we can define the Euler characteristic
(using the rank in the case of abelian groups and the Hamel dimension in the case of vector spaces). It turns out that the Euler characteristic can also be computed on the level of homology:
and, especially in algebraic topology, this provides two ways to compute the important invariant for the object X which gave rise to the chain complex.
Every short exact sequence
of chain complexes gives rise to a long exact sequence of homology groups
All maps in this long exact sequence are induced by the maps between the chain complexes, except for the maps The latter are called connecting homomorphisms and are provided by the zigzag lemma. This lemma can be applied to homology in numerous ways that aid in calculating homology groups, such as the theories of relative homology and MayerVietoris sequences.
Applications
Application in pure mathematics
Notable theorems proved using homology include the following:
 The Brouwer fixed point theorem: If f is any continuous map from the ball B^{n} to itself, then there is a fixed point with
 Invariance of domain: If U is an open subset of and is an injective continuous map, then is open and f is a homeomorphism between U and V.
 The Hairy ball theorem: any continuous vector field on the 2sphere (or more generally, the 2ksphere for any ) vanishes at some point.
 The Borsuk–Ulam theorem: any continuous function from an nsphere into Euclidean nspace maps some pair of antipodal points to the same point. (Two points on a sphere are called antipodal if they are in exactly opposite directions from the sphere's center.)
 Invariance of dimension: if nonempty open subsets and are homeomorphic, then ^{[24]}
Application in science and engineering
In topological data analysis, data sets are regarded as a point cloud sampling of a manifold or algebraic variety embedded in Euclidean space. By linking nearest neighbor points in the cloud into a triangulation, a simplicial approximation of the manifold is created and its simplicial homology may be calculated. Finding techniques to robustly calculate homology using various triangulation strategies over multiple length scales is the topic of persistent homology.^{[25]}
In sensor networks, sensors may communicate information via an adhoc network that dynamically changes in time. To understand the global context of this set of local measurements and communication paths, it is useful to compute the homology of the network topology to evaluate, for instance, holes in coverage.^{[26]}
In dynamical systems theory in physics, Poincaré was one of the first to consider the interplay between the invariant manifold of a dynamical system and its topological invariants. Morse theory relates the dynamics of a gradient flow on a manifold to, for example, its homology. Floer homology extended this to infinitedimensional manifolds. The KAM theorem established that periodic orbits can follow complex trajectories; in particular, they may form braids that can be investigated using Floer homology.^{[27]}
In one class of finite element methods, boundaryvalue problems for differential equations involving the HodgeLaplace operator may need to be solved on topologically nontrivial domains, for example, in electromagnetic simulations. In these simulations, solution is aided by fixing the cohomology class of the solution based on the chosen boundary conditions and the homology of the domain. FEM domains can be triangulated, from which the simplicial homology can be calculated.^{[28]}^{[29]}
Software
Various software packages have been developed for the purposes of computing homology groups of finite cell complexes. Linbox is a C++ library for performing fast matrix operations, including Smith normal form; it interfaces with both Gap and Maple. Chomp, CAPD::Redhom and Perseus are also written in C++. All three implement preprocessing algorithms based on simplehomotopy equivalence and discrete Morse theory to perform homologypreserving reductions of the input cell complexes before resorting to matrix algebra. Kenzo is written in Lisp, and in addition to homology it may also be used to generate presentations of homotopy groups of finite simplicial complexes. Gmsh includes a homology solver for finite element meshes, which can generate Cohomology bases directly usable by finite element software.^{[28]}
See also
 Betti number
 Cycle space
 De Rham cohomology
 Eilenberg–Steenrod axioms
 Extraordinary homology theory
 Homological algebra
 Homological conjectures in commutative algebra
 Homological connectivity
 Homological dimension
 Homotopy group
 Künneth theorem
 List of cohomology theories  also has a list of homology theories
 Poincaré duality
Notes
 ^ in part from Greek ὁμός homos "identical"
 ^ Stillwell 1993, p. 170
 ^ Weibel 1999, pp. 2–3 (in PDF)
 ^ ^{a} ^{b} Richeson 2008, p. 254
 ^ ^{a} ^{b} Weeks, Jeffrey R. (2001). The Shape of Space. CRC Press. ISBN 9780203912669.
 ^ Richeson 2008
 ^ Richeson 2008, p. 258
 ^ Weibel 1999, p. 4
 ^ Hilton 1988, p. 284
 ^ For example L'émergence de la notion de groupe d'homologie, Nicolas Basbois (PDF), in French, note 41, explicitly names Noether as inventing the homology group.
 ^ Hirzebruch, Friedrich, Emmy Noether and Topology in Teicher 1999, pp. 61–63.
 ^ Bourbaki and Algebraic Topology by John McCleary (PDF) Archived 20080723 at the Wayback Machine gives documentation (translated into English from French originals).
 ^ Richeson 2008, p. 264
 ^ Spanier 1966, p. 155
 ^ ^{a} ^{b} ^{c} Gowers, BarrowGreen & Leader 2010, pp. 390–391
 ^ Wildberger, Norman J. (2012). "More homology computations". YouTube. Archived from the original on 20211211.
 ^ ^{a} ^{b} Hatcher 2002, p. 106
 ^ Wildberger, Norman J. (2012). "Delta complexes, Betti numbers and torsion". YouTube. Archived from the original on 20211211.
 ^ Hatcher 2002, pp. 105–106
 ^ Hatcher 2002, p. 113
 ^ Hatcher 2002, p. 110
 ^ Wildberger, N. J. (2012). "An introduction to homology". YouTube. Archived from the original on 20211211.
 ^ Spanier 1966, p. 156
 ^ Hatcher 2002, p. 126.
 ^ "CompTop overview". Retrieved 16 March 2014.
 ^ "Robert Ghrist: applied topology". Retrieved 16 March 2014.
 ^ van den Berg, J.B.; Ghrist, R.; Vandervorst, R.C.; Wójcik, W. (2015). "Braid Floer homology" (PDF). Journal of Differential Equations. 259 (5): 1663–1721. Bibcode:2015JDE...259.1663V. doi:10.1016/j.jde.2015.03.022. S2CID 16865053.
 ^ ^{a} ^{b} Pellikka, M; S. Suuriniemi; L. Kettunen; C. Geuzaine (2013). "Homology and Cohomology Computation in Finite Element Modeling" (PDF). SIAM J. Sci. Comput. 35 (5): B1195–B1214. CiteSeerX 10.1.1.716.3210. doi:10.1137/130906556.
 ^ Arnold, Douglas N.; Richard S. Falk; Ragnar Winther (16 May 2006). "Finite element exterior calculus, homological techniques, and applications". Acta Numerica. 15: 1–155. Bibcode:2006AcNum..15....1A. doi:10.1017/S0962492906210018. S2CID 122763537.
References
 Cartan, Henri Paul; Eilenberg, Samuel (1956). Homological Algebra. Princeton mathematical series. Vol. 19. Princeton University Press. ISBN 9780674079779. OCLC 529171.
 Eilenberg, Samuel; Moore, J.C. (1965). Foundations of relative homological algebra. Memoirs of the American Mathematical Society number. Vol. 55. American Mathematical Society. ISBN 9780821812556. OCLC 1361982.
 Gowers, Timothy; BarrowGreen, June; Leader, Imre, eds. (2010), The Princeton Companion to Mathematics, Princeton University Press, ISBN 9781400830398.
 Hatcher, A. (2002), Algebraic Topology, Cambridge University Press, ISBN 0521795400. Detailed discussion of homology theories for simplicial complexes and manifolds, singular homology, etc.
 Hilton, Peter (1988), "A Brief, Subjective History of Homology and Homotopy Theory in This Century", Mathematics Magazine, Mathematical Association of America, 60 (5): 282–291, doi:10.1080/0025570X.1988.11977391, JSTOR 2689545
 Richeson, D. (2008), Euler's Gem: The Polyhedron Formula and the Birth of Topology, Princeton University.
 Spanier, Edwin H. (1966), Algebraic Topology, Springer, p. 155, ISBN 0387906460.
 Stillwell, John (1993), Classical Topology and Combinatorial Group Theory, Springer, doi:10.1007/9781461243724_6, ISBN 9780387979700.
 Teicher, M., ed. (1999), The Heritage of Emmy Noether, Israel Mathematical Conference Proceedings, BarIlan University/American Mathematical Society/Oxford University Press, ISBN 9780198510451, OCLC 223099225
 Weibel, Charles A. (1999), "28. History of Homological Algebra" (PDF), in James, I. M. (ed.), History of Topology, Elsevier, ISBN 9780080534077.
External links
 Homology group at Encyclopaedia of Mathematics
 [1] N.J. Windberger intro to algebraic topology, last six lectures with an easy intro to homology
 [2] Algebraic topology Allen Hatcher  Chapter 2 on homology