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A branch of algebraic topology concerning the study of cocycles and coboundaries. It is in some sense a dual theory to homology theory. This tag can be further specialized by using it in conjunction with the tags group-cohomology, etale-cohomology, sheaf-cohomology, galois-cohomology, lie-algebra-cohomology, motivic-cohomology, equivariant-cohomology, ...

10 votes

How are these algebraic and geometric notions of homotopy of maps between manifolds related?

It's very hard to go back since an algebraic homotopy is very weak information, it exists if and only if $f$ and $g$ induce the same map on de Rham cohomology (since these are chain complexes over a field …
Peter Teichner's user avatar
7 votes
Accepted

Is the following map from Z(G) x H^3(G, C*) --> H^2(G, C*) ever nontrivial?

The proof is easy since the cohomology rings are polynomial, respectively exterior, algebras. …
Peter Teichner's user avatar
21 votes
Accepted

Complex orientations on homotopy

For example, if E=HZ is ordinary Z-cohomology then X=BSO works, if E=KO then X=BSpin works, if E=KU then X=BU or X=BSpin$^c$ works etc. … In the case X=BU this is called a complex orientation of E and has been studied extensively because it simplifies computations of E-cohomology tremendously. …
Peter Teichner's user avatar
18 votes

Explanation for the Chern character

The hard part is to explain why domain and range of this map are K-theory respectively de Rham cohomology. …
Peter Teichner's user avatar