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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, ...

13 votes
2 answers
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Image of a map on cohomology rings

$ I have a particular $X,Y$ in mind (the inclusion of one compact complex manifold into another, each with even-degree cohomology) but I'm hoping phrases like "Postnikov tower", "cofibrant replacement …
Allen Knutson's user avatar
18 votes

Why study Higher Sheaf Cohomology?

In the context of coherent sheaves, very often the approach is to compute Euler characteristic, give some reason that higher sheaf cohomology vanishes, and conclude that one has computed $H^0$. … Serre gave a talk (around 1998) in which he explained that "It was once believed that the only good sheaf cohomology is dead sheaf cohomology" but "nowadays this is not the [politically] correct view." …
Allen Knutson's user avatar
2 votes

Is there a presentation of the cohomology of the moduli stack of torsion sheaves on an ellip...

That computes T^m-equivariant cohomology, in which the GL(m)-equivariant sits as the S_m-invariant part. …
Allen Knutson's user avatar
1 vote

Linear sections of $Gr(V,2)$

Pull back the equation $[W] = [H]^{codim\ W} \in \mathbb P^N$. This cohomological pullback is computable set-theoretically, as $[W\cap G]$, if the intersection is transverse.
Allen Knutson's user avatar
2 votes
Accepted

Schubert Polynomials for Complex Projective Space

In general for a Grassmannian $Gr_k(\mathbb C^n)$, the Schubert polynomials are Schur polynomials in $k$ variables, one for each partition $\lambda$ in a $k\times (n-k)$ rectangle. In this case $S_\la …
Allen Knutson's user avatar
5 votes

On the cohomology ring of the Grassmannian

The Schubert classes on $G/P$ are the classes of the Schubert varieties, which are the closures of the Schubert cells, each of which contains a unique $T$-fixed point. The $T$-fixed points on $G/P$ ar …
Allen Knutson's user avatar