This is a list of some of the ordinary and generalized (or extraordinary) homology and cohomology theories in algebraic topology that are defined on the categories of CW complexes or spectra. For other sorts of homology theories see the links at the end of this article.
If is a spectrum, then it defines generalized homology and cohomology theories on the category of spectra as follows:
These are the theories satisfying the "dimension axiom" of the EilenbergâÂÂSteenrod axioms that the homology of a point vanishes in dimension other than 0. They are determined by an abelian coefficient group , and denoted by (where is sometimes omitted, especially if it is ). Usually is the integers, the rationals, the reals, the complex numbers, or the integers mod a prime .
The cohomology functors of ordinary cohomology theories are represented by EilenbergâÂÂMacLane spaces.
On simplicial complexes, these theories coincide with singular homology and cohomology.
Spectrum: (EilenbergâÂÂMacLane spectrum of the integers.)
Coefficient ring: if , otherwise.
The original homology theory.
Spectrum: (EilenbergâÂÂMac Lane spectrum of the rationals.)
Coefficient ring: if , otherwise.
These are the easiest of all homology theories. The homology groups are often denoted by . The homology groups , , with rational, real, and complex coefficients are all similar, and are used mainly when torsion is not of interest (or too complicated to work out). The Hodge decomposition writes the complex cohomology of a complex projective variety as a sum of sheaf cohomology groups.
Spectrum: (EilenbergâÂÂMaclane spectrum of the integers mod .)
Coefficient ring: (integers mod ) if , otherwise.
The simpler K-theories of a space are often related to vector bundles over the space, and different sorts of K-theories correspond to different structures that can be put on a vector bundle.
Spectrum: KO
Coefficient ring: The coefficient groups ÃÂ<sub>i</sub>(KO) have period 8 in i, given by the sequence Z, Z<sub>2</sub>, Z<sub>2</sub>,0, Z, 0, 0, 0, repeated. As a ring, it is generated by a class ÷ in degree 1, a class x<sub>4</sub> in degree 4, and an invertible class v<sub>1</sub><sup>4</sup> in degree 8, subject to the relations that 2÷ = ÷<sup>3</sup> = ÷x<sub>4</sub> = 0, and x<sub>4</sub><sup>2</sup> = 4v<sub>1</sub><sup>4</sup>.
KO<sup>0</sup>(X) is the ring of stable equivalence classes of real vector bundles over X. Bott periodicity implies that the K-groups have period 8.
Spectrum: KU (even terms BU or Z ÃÂ BU, odd terms U).
Coefficient ring: The coefficient ring K<sup>*</sup>(point) is the ring of Laurent polynomials in a generator of degree 2.
K<sup>0</sup>(X) is the ring of stable equivalence classes of complex vector bundles over X. Bott periodicity implies that the K-groups have period 2.
Spectrum: KSp
Coefficient ring: The coefficient groups ÃÂ<sub>i</sub>(KSp) have period 8 in i, given by the sequence Z, 0, 0, 0,Z, Z<sub>2</sub>, Z<sub>2</sub>,0, repeated.
KSp<sup>0</sup>(X) is the ring of stable equivalence classes of quaternionic vector bundles over X. Bott periodicity implies that the K-groups have period 8.
Spectrum: KG
G is some abelian group; for example the localization Z<sub>(p)</sub> at the prime p. Other K-theories can also be given coefficients.
Spectrum: KSC
Coefficient ring: to be written...
The coefficient groups (KSC) have period 4 in i, given by the sequence Z, Z<sub>2</sub>, 0, Z, repeated. Introduced by Donald W. Anderson in his unpublished 1964 University of California, Berkeley Ph.D. dissertation, "A new cohomology theory".
Spectrum: ku for connective K-theory, ko for connective real K-theory.
Coefficient ring: For ku, the coefficient ring is the ring of polynomials over Z on a single class v<sub>1</sub> in dimension 2. For ko, the coefficient ring is the quotient of a polynomial ring on three generators, ÷ in dimension 1, x<sub>4</sub> in dimension 4, and v<sub>1</sub><sup>4</sup> in dimension 8, the periodicity generator, modulo the relations that 2÷ = 0, x<sub>4</sub><sup>2</sup> = 4v<sub>1</sub><sup>4</sup>, ÷<sup>3</sup> = 0, and ÷x = 0.
Roughly speaking, this is K-theory with the negative dimensional parts killed off.
This is a cohomology theory defined for spaces with involution, from which many of the other K-theories can be derived.
Cobordism studies manifolds, where a manifold is regarded as "trivial" if it is the boundary of another compact manifold. The cobordism classes of manifolds form a ring that is usually the coefficient ring of some generalized cohomology theory. There are many such theories, corresponding roughly to the different structures that one can put on a manifold.
The functors of cobordism theories are often represented by Thom spaces of certain groups.
Spectrum: S (sphere spectrum).
Coefficient ring: The coefficient groups ÃÂ<sub>n</sub>(S) are the stable homotopy groups of spheres, which are notoriously hard to compute or understand for n > 0. (For n < 0 they vanish, and for n = 0 the group is Z.)
Stable homotopy is closely related to cobordism of framed manifolds (manifolds with a trivialization of the normal bundle).
Spectrum: MO (Thom spectrum of orthogonal group)
Coefficient ring: ÃÂ<sub>*</sub>(MO) is the ring of cobordism classes of unoriented manifolds, and is a polynomial ring over the field with 2 elements on generators of degree i for every i not of the form 2<sup>n</sup>âÂÂ1. That is: where can be represented by the classes of while for odd indices one can use appropriate Dold manifolds.
Unoriented bordism is 2-torsion, since 2M is the boundary of .
MO is a rather weak cobordism theory, as the spectrum MO is isomorphic to H(ÃÂ<sub>*</sub>(MO)) ("homology with coefficients in ÃÂ<sub>*</sub>(MO)") â MO is a product of EilenbergâÂÂMacLane spectra. In other words, the corresponding homology and cohomology theories are no more powerful than homology and cohomology with coefficients in Z/2Z. This was the first cobordism theory to be described completely.
Spectrum: MU (Thom spectrum of unitary group)
Coefficient ring: ÃÂ<sub>*</sub>(MU) is the polynomial ring on generators of degree 2, 4, 6, 8, ... and is naturally isomorphic to Lazard's universal ring, and is the cobordism ring of stably almost complex manifolds.
Spectrum: MSO (Thom spectrum of special orthogonal group)
Coefficient ring: The oriented cobordism class of a manifold is completely determined by its characteristic numbers: its StiefelâÂÂWhitney numbers and Pontryagin numbers, but the overall coefficient ring, denoted is quite complicated. Rationally, and at 2 (corresponding to Pontryagin and StiefelâÂÂWhitney classes, respectively), MSO is a product of EilenbergâÂÂMacLane spectra â and â but at odd primes it is not, and the structure is complicated to describe. The ring has been completely described integrally, due to work of John Milnor, Boris Averbuch, Vladimir Rokhlin, and C. T. C. Wall.
Spectrum: MSU (Thom spectrum of special unitary group)
Coefficient ring:
Spectrum: MSpin (Thom spectrum of spin group)
Coefficient ring: See .
Spectrum: MSp (Thom spectrum of symplectic group)
Coefficient ring:
Spectrum: MPL, MSPL, MTop, MSTop
Coefficient ring:
The definition is similar to cobordism, except that one uses piecewise linear or topological instead of smooth manifolds, either oriented or unoriented. The coefficient rings are complicated.
Spectrum: BP
Coefficient ring: ÃÂ<sub>*</sub>(BP) is a polynomial algebra over Z<sub>(p)</sub> on generators v<sub>n</sub> of dimension 2(p<sup>n</sup> â 1) for n âÂÂ¥ 1.
BrownâÂÂPeterson cohomology BP is a summand of MU<sub>p</sub>, which is complex cobordism MU localized at a prime p. In fact MU<sub>(p)</sub> is a sum of suspensions of BP.
Spectrum: K(n) (They also depend on a prime p.)
Coefficient ring: F<sub>p</sub>[v<sub>n</sub>, v<sub>n</sub><sup>âÂÂ1</sup>], where v<sub>n</sub> has degree 2(p<sup>n</sup> -1).
These theories have period 2(p<sup>n</sup> â 1). They are named after Jack Morava.
Spectrum E(n)
Coefficient ring Z<sub>(2)</sub>[v<sub>1</sub>, ..., v<sub>n</sub>, 1/v<sub>n</sub>] where v<sub>i</sub> has degree 2(2<sup>i</sup>−1)
Spectrum:
Coefficient ring:
Spectrum: Ell
Spectra: tmf, TMF (previously called eo<sub>2</sub>.)
The coefficient ring ÃÂ<sub>*</sub>(tmf) is called the ring of topological modular forms. TMF is tmf with the 24th power of the modular form ÃÂ inverted, and has period 24<sup>2</sup>=576. At the prime p = 2, the completion of tmf is the spectrum eo<sub>2</sub>, and the K(2)-localization of tmf is the Hopkins-Miller Higher Real K-theory spectrum EO<sub>2</sub>.