# Cyclic cohomology

Cyclic cohomology was developed as a replacement of the de Rham theory in the context of non-commutative algebras. It was discovered independently by B. Feigin and B. Tsygan as a non-commutative analogue of de Rham cohomology and by A. Connes as the cohomological structure involved in the computation of indices of elliptic operators (cf. Index formulas) and the range of a Chern character defined on -homology (cf. also -theory). It plays a fundamental role in various generalizations of index theorems to foliations and coverings (higher indices of Connes and H. Moscovici). Among the more classical applications there are the proof of the Novikov conjecture for a large class of groups and local formulas for Pontryagin classes.

Suppose that is a unital algebra over a field of characteristic zero. Let

The complex computes the Hochschild cohomology of with values in the dual -bimodule . The subspace of cyclic cochains,

is closed under the Hochschild coboundary mapping and the cohomology of the induced complex is called the cyclic cohomology of the algebra , denoted by . This is a contravariant functor from the category of associative algebras to the category of linear spaces. The inclusion of complexes

gives a long exact sequence of cohomology which, in this case, has the form

and is usually called the Connes–Gysin exact sequence. The induced spectral sequence is one of the main tools for the computation of cyclic cohomology. The periodicity operator gives rise to the stabilized version (-graded) of cyclic cohomology given by the direct limit

and called periodic cyclic cohomology. Both cyclic and periodic cyclic cohomologies admit external products:

Some examples are:

Any tracial functional (i.e. for all ) on the algebra defines an element of .

Let be a smooth, closed manifold and . Restricting to the continuous (in the -topology) cochains, one finds

where is the space of de Rham -currents on and the operator coincides with the transpose of the de Rham differential . In particular, the periodic cyclic cohomology of coincides with de Rham homology of . An explicit mapping is given by

The definitions given above can be easily extended to non-unital algebras.

## Properties of periodic cyclic cohomology.

Stability. The mapping

is an isomorphism. This holds also for (non-periodic)) cyclic cohomology. Here, is the mapping induced by from the algebra of -matrices over to .

Excision. Given an exact sequence of algebras

there exists an associated six-term exact sequence

The excision holds also in the case of Fréchet algebras (cf. also Fréchet algebra) if the quotient mapping has a continuous (linear) lifting. For cyclic cohomology, under a certain condition on the ideal (called -unitality) there exists a corresponding long exact sequence.

Homotopy invariance. Suppose that, in the context of topological algebras and continuous cochains, a differentiable family of homomorphisms is given:

The corresponding mappings on the periodic cyclic cohomology are -independent. In the context of cyclic cohomology the corresponding result says that inner derivations act trivially.

Pairing with -theory. Suppose that is a Banach algebra and that is a dense subalgebra closed under holomorphic functional calculus in . Suppose that has been given a Fréchet topology under which its imbedding into is continuous. Restricting to continuous cochains, there exists a pairing

On the level of idempotents , this pairing is given by

This pairing is consistent with the six-term exact sequences in periodic cyclic cohomology and -theory.

Bott periodicity. In the topological context, let be the projective tensor product of with the algebra of rapidly decreasing functions on . Let denote the cyclic cocycle on given by the current

The mapping

is an isomorphism.

#### References

[a1] | A. Connes, "Noncommutative geometry" , Acad. Press (1994) |

[a2] | J. Cuntz, D. Quillen, "Algebra extensions and nonsingularity" J. Amer. Math. Soc. , 8 , Amer. Math. Soc. (1995) pp. 251–289 |

[a3] | J-L. Loday, "Cyclic homology" , Springer (1991) |

**How to Cite This Entry:**

Cyclic cohomology.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Cyclic_cohomology&oldid=11855