In algebraic topology, a branch of mathematics, singular homology refers to the study of a certain set of algebraic invariants of a topological space X, the so-called homology groups
Contents
- Singular simplices
- Singular chain complex
- Homotopy invariance
- Functoriality
- Coefficients in R
- Relative homology
- Cohomology
- Betti homology and cohomology
- Extraordinary homology
- References
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 - induces the singular chain complex. The singular homology is then the homology of the chain complex. The resulting homology groups are the same for all homotopically equivalent spaces, which is the reason for their study. These constructions can be applied to all topological spaces, and so singular homology can be expressed in terms of category theory, where the homology group becomes a functor from the category of topological spaces to the category of graded abelian groups. These ideas are developed in greater detail below.
Singular simplices
A singular n-simplex is a continuous mapping
The boundary of
corresponding to the vertices
is a formal sum of the faces of the simplex image designated in a specific way. (That is, a particular face has to be the image of
Singular chain complex
The usual construction of singular homology proceeds by defining formal sums of simplices, which may be understood to be elements of a free abelian group, 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
The boundary
is a homomorphism of groups. The boundary operator, together with the
The kernel of the boundary operator is
It can also be shown that
The elements of
Homotopy invariance
If X and Y are two topological spaces with the same homotopy type, then
for all n ≥ 0. This means homology groups are topological invariants.
In particular, if X is a connected contractible space, then all its homology groups are 0, except
A proof for the homotopy invariance of singular homology groups can be sketched as follows. A continuous map f: X → Y induces a homomorphism
It can be verified immediately that
i.e. f# is a chain map, which descends to homomorphisms on homology
We now show that if f and g are homotopically equivalent, then f* = g*. From this follows that if f is a homotopy equivalence, then f* is an isomorphism.
Let F : X × [0, 1] → Y be a homotopy that takes f to g. On the level of chains, define a homomorphism
that, geometrically speaking, takes a basis element σ: Δn → X of Cn(X) to the "prism" P(σ): Δn × I → Y. The boundary of P(σ) can be expressed as
So if α in Cn(X) is an n-cycle, then f#(α ) and g#(α) differ by a boundary:
i.e. they are homologous. This proves the claim.
Functoriality
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 of category theory. In particular, the homology group can be understood to be a functor from the category of topological spaces Top to the category of abelian groups Ab.
Consider first that
by defining
where
from the category of topological spaces to the category of abelian groups.
The boundary operator commutes with continuous maps, so that
from the category of topological spaces to the category of abelian groups. By the homotopy axiom, one has that
This distinguishes singular homology from other homology theories, wherein
More generally, the homology functor is defined axiomatically, as a functor on an abelian category, or, alternately, as a functor on chain complexes, satisfying axioms that require a boundary morphism that turns short exact sequences into long exact sequences. 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
which maps topological spaces as
The second, algebraic part is the homology functor
which maps
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 hComp or K, the homotopy category of chain complexes.
Coefficients in R
Given any unital ring 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
which is now an R-module. Of course, it is usually not a free module. The usual homology group is regained by noting that
when one takes the ring to be the ring of integers. The notation Hn(X, R) should not be confused with the nearly identical notation Hn(X, A), which denotes the relative homology (below).
Relative homology
For a subspace
where the quotient of chain complexes is given by the short exact sequence
Cohomology
By dualizing the homology chain complex (i.e. applying the functor Hom(-, R), R being any ring) we obtain a cochain complex with coboundary map
The cohomology groups have a richer, or at least more familiar, algebraic structure than the homology groups. Firstly, they form a differential graded algebra as follows:
There are additional cohomology operations, 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 Category:Homology theory), the terms Betti homology and Betti cohomology are sometimes applied (particularly by authors writing on algebraic geometry) to the singular theory, as giving rise to the Betti numbers of the most familiar spaces such as simplicial complexes and closed manifolds.
Extraordinary homology
If one defines a homology theory axiomatically (via the Eilenberg–Steenrod axioms), and then relaxes one of the axioms (the dimension axiom), one obtains a generalized theory, called an extraordinary homology theory. These originally arose in the form of extraordinary cohomology theories, namely K-theory and cobordism theory. In this context, singular homology is referred to as ordinary homology.