# Homology (mathematics)

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Homology (mathematics)

In mathematics (especially algebraic topology and abstract algebra), homology (in Greek ὁμός "homos" "identical") is a certain general procedure to associate a sequence of abelian groups or modules with a given mathematical object such as a topological space or a group. See homology theory for more background, or singular homology for a concrete version for topological spaces, or group cohomology for a concrete version for groups.

For a topological space, the homology groups are generally much easier to compute than the homotopy groups, and consequently one usually will have an easier time working with homology to aid in the classification of spaces.

Construction of homology groups

The procedure works as follows: Given an object such as a topological space $X$, one first defines a "chain complex" $A=C\left(X\right)$ that encodes information about $X$. A chain complex is a sequence of abelian groups or modules $A_0, A_1, A_2, dots$ connected by homomorphisms $d_n : A_n ightarrow A_\left\{n-1\right\}$, such that the composition of any two consecutive maps is zero: $d_n circ d_\left\{n+1\right\} = 0$ for all "n". This means that the image of the "n"+1-th map is contained in the kernel of the "n"-th, and we can define the "n"-th homology group of "X" to be the factor group (or quotient module)

:$H_n\left(X\right) = ker\left(d_n\right) / mathrm\left\{Im\right\}\left(d_\left\{n+1\right\}\right)$

The standard notation is $ker\left(d_n\right)=Z_n\left(X\right)$ and $operatorname\left\{im\right\}\left(d_\left\{n+1\right\}\right)=B_n\left(X\right)$. Note that the computation of these two groups is usually rather difficult, since they are very large groups. On the other hand, machinery exists that allows one to compute the corresponding homology group easily.

The "simplicial homology" groups $H_n\left(X\right)$ of a "simplicial complex" $X$ are defined using the simplicial chain complex $C\left(X\right)$,with $C\left(X\right)_n$ the free abelian group generated by the $n$-simplices of $X$. The "singular homology" groups $H_n\left(X\right)$ are defined for any topological space $X$, and agree with the simplicial homology groups for a simplicial complex.

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 "n"th map. The homology groups of $X$ therefore measure "how far" the chain complex associated to $X$ is from being exact.

Cohomology groups are formally similar: one starts with a cochain complex, which is the same as a chain complex but whose arrows, now denoted $d^n$ point in the direction of increasing "n" rather than decreasing "n"; then the groups $ker\left(d^n\right) = Z^n\left(X\right)$ and $operatorname\left\{im\right\}\left(d^\left\{n - 1\right\}\right) = B^n\left(X\right)$ follow from the same description and:$H^n\left(X\right) = Z^n\left(X\right)/B^n\left(X\right)$, as before.

Examples

The motivating example comes from algebraic topology: the simplicial homology of a simplicial complex $X$. Here $A_n$ is the free abelian group or module whose generators are the "n"-dimensionaloriented simplexes of $X$. The mappings are called the "boundary mappings" and send the simplex with vertices

:$\left(a \left[0\right] , a \left[1\right] , dots, a \left[n\right] \right)$

to the sum

:$sum_\left\{i=0\right\}^n \left(-1\right)^i\left(a \left[0\right] , dots, a \left[i-1\right] , a \left[i+1\right] , dots, a \left[n\right] \right)$

(which is considered $0$ if $n=0$).

If we take the modules to be over a field, then the dimensionof the "n"-th homology of $X$ turns out to be the number of "holes" in $X$ at dimension "n".

Using this example as a model, one can define a singular homology for any topological space $X$. We define a chain complex for $X$ by taking $A_n$ to be the free abelian group (or free module) whose generators are all continuous maps from "n"-dimensional simplices into $X$. The homomorphisms $d_n$ arise from the boundary maps of simplices.

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 $F_1$ and a surjective homomorphism $p_1 : F_1 ightarrow X$. Then one finds a free module $F_2$ and a surjective homomorphism $p_2 : F_2 ightarrow mathrm\left\{ker\right\}\left(p_1\right)$. Continuing in this fashion, a sequence of free modules $F_n$ and homomorphisms $p_n$ can be defined. By applying the functor $F$ to this sequence, one obtains a chain complex; the homology $H_n$ of this complex depends only on $F$ and $X$ and is, by definition, the "n"-th derived functor of $F$, applied to $X$.

Homology functors

Chain complexes form a category: A morphism from the chain complex $\left(d_n colon A_n ightarrow A_\left\{n-1\right\}\right)$ to the chain complex $\left(e_ncolon B_n ightarrow B_\left\{n-1\right\}\right)$ is a sequence of homomorphisms $f_ncolon A_n ightarrow B_n$ such that $f_\left\{n-1\right\} circ d_n = e_\left\{n\right\} circ f_n$ for all "n". The "n"-th homology "Hn" 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 "X → Y" induces a morphism from the chain complex of "X" to the chain complex of "Y"), then the "Hn" 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 "Hn") form "contravariant" functors from the category that "X" belongs to into the category of abelian groups or modules.

Properties

If $\left(d_n : A_n ightarrow A_\left\{n-1\right\}\right)$ is a chain complex such that all but finitely many $A_n$ are zero, and the others are finitely generated abelian groups (or finite dimensional vector spaces), then we can define the "Euler characteristic"

:$chi = sum \left(-1\right)^n , mathrm\left\{rank\right\}\left(A_n\right)$

(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:

:$chi = sum \left(-1\right)^n , mathrm\left\{rank\right\}\left(H_n\right)$

and, especially in algebraic topology, this provides two ways to compute the important invariant $chi$ for the object $X$ which gave rise to the chain complex.

Every short exact sequence

:$0 ightarrow A ightarrow B ightarrow C ightarrow 0$

of chain complexes gives rise to a long exact sequence of homology groups

:$cdots ightarrow H_n\left(A\right) ightarrow H_n\left(B\right) ightarrow H_n\left(C\right) ightarrow H_\left\{n-1\right\}\left(A\right) ightarrow H_\left\{n-1\right\}\left(B\right) ightarrow H_\left\{n-1\right\}\left(C\right) ightarrow H_\left\{n-2\right\}\left(A\right) ightarrow cdots ,$

All maps in this long exact sequence are induced by the maps between the chain complexes, except for the maps $H_n\left(C\right) ightarrow H_\left\{n-1\right\}\left(A\right).$ The latter are called "connecting homomorphisms" and are provided by the snake lemma.

*Simplicial homology
*Singular homology
*Homology theory
*Homological algebra

References

* Cartan, Henri Paul and Eilenberg, Samuel (1956) "Homological Algebra" Princeton University Press, Princeton, NJ, [http://worldcat.org/oclc/529171 OCLC 529171]
* Eilenberg, Samuel and Moore, J. C. (1965) "Foundations of relative homological algebra" (Memoirs of the American Mathematical Society number 55) American Mathematical Society, Providence, R.I., [http://worldcat.org/oclc/1361982 OCLC 1361982]
* Hatcher, A., (2002) " [http://www.math.cornell.edu/~hatcher/AT/ATchapters.html Algebraic Topology] " Cambridge University Press, ISBN 0-521-79540-0. Detailed discussion of homology theories for simplicial complexes and manifolds, singular homology, etc.
*planetmath reference|id=3720|title=Homology (Topological space)

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