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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, ...

44 votes
4 answers
5k views

Integral cohomology (stable) operations

There have been a couple questions on MO, and elsewhere, that have made me curious about integral or rational cohomology operations. … So my questions are essentially as follows: 1) Are there any "interesting" rational cohomology operations? …
5 votes

Formal Group Laws on Ring Spectra?

I think the right correspondence is the one mentioned in the comments. The main reason is that if you have a map of ring spectra $HL \to E$ then $E$ is an Eilenberg-Maclane spectrum. So in particular, …
Sean Tilson's user avatar
  • 3,726
4 votes

Are the homology and cohomology Serre spectral sequences dual to each other?

What is true though, is that if X is an H-space, then the differential in the homology Serre SS is a derivation like it is in cohomology. …
Sean Tilson's user avatar
  • 3,726
11 votes
Accepted

Examples of computing Ext and Tor functors?

$\mathbb{Q}/\mathbb{Z}$ is a pretty terrible abelian group, or a rather hard one, there may be better injective resolutions to work with. It would certainly be easier to do the projective resolution, …
Sean Tilson's user avatar
  • 3,726
15 votes
1 answer
1k views

Complex orientations on homotopy

I am wondering if there is a more "geometric" formulation of complex orientations for cohomology theories than just a computation of $E^*\mathbb{C}$P$^{\infty}$ or a statement about Thom classes. … how much of these $E$-orientations of X is detected by E-cohomology of X? …