Stein manifold

In mathematics, sheaf cohomology is the aspect of sheaf theory, concerned with sheaves of abelian groups, that applies homological algebra to make possible effective calculation of the global sections of a sheaf F. This is the main step, in numerous areas, from sheaf theory as a description of a geometric problem, to its use as a tool capable of calculating dimensions of important geometric invariants.

Its development was rapid in the years after 1950, when it was realised that sheaf cohomology was connected with more classical methods applied to the Riemann-Roch theorem, the analysis of a linear system of divisors in algebraic geometry, several complex variables, and Hodge theory. The dimensions or ranks of sheaf cohomology groups became a fresh source of geometric data, or gave rise to new interpretations of older work.

Definitions

The approach of Čech cohomology

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The first version of sheaf cohomology to be defined was that based on Čech cohomology, in which the relatively small change was made of attributing to an open set U of a topological space X an abelian group F(U) that 'varies' with U, rather than an abelian group A that is fixed ahead of time. This means that cochains are easy to write down rather concretely; in fact the model applications, such as the Cousin problems on meromorphic functions, stay within fairly familiar mathematical territory. From the sheaf point of view, the Čech theory is the restriction to sheaves of locally constant functions with values in A. Within sheaf theory it is easy to see that 'twisted' versions, with local coefficients on which the fundamental group acts, are also subsumed — along with some very different sorts of more general coefficients.

One problem with that theory was that Čech cohomology itself fails to have good properties, unless X itself is well-behaved. This is not a difficulty in case X is something like a manifold; but embarrassing for applications to algebraic geometry, since the Zariski topology is in general not Hausdorff. The problem with the Čech theory manifests itself in the failure of the long exact sequence of cohomology groups associated to a short exact sequence of sheaves. This in practice is the basic method of attacking a calculation (i.e. to show how a given sheaf is involved with others in a short exact sequence, and draw consequences). The theory stood in this state of disarray only for a short while: Jean-Pierre Serre showed that the Čech theory worked, and on the other hand Alexandre Grothendieck proposed a more abstract definition that would build in the long exact sequence.

Definition by derived functors

The Grothendieck definition clarified the status of sheaf cohomology of a topological space X with coefficients in a sheaf $\mathcal{ℱ}$ as the right derived functor of the global section functor:

${\mathrm{\Gamma }}_X:\mathcal{ℱ}\mapsto \mathcal{ℱ}(X).$

This functor is not an exact functor, a fact familiar in other terms from the theory of branch cuts (for example, in the case of the logarithm of a complex number: see exponential sequence). It is a left exact functor, and therefore has a sequence of right derived functors, denoted by

$H^i(X,\mathcal{ℱ}),i\ge 0.$

The existence of these derived functors is supplied by homological algebra of the abelian category of sheaves (and indeed this was a main reason to set up that theory). It depends on having injective resolutions; that is, in theory calculations can be done with injective resolutions, though in practice short and long exact sequences may be a better idea.

Because the derived functor can be computed by applying the functor to any acyclic resolution and keeping the cohomology of the complex, there are a number of other ways to compute cohomology groups. Depending on the concrete situation, fine, flasque, soft or acyclic sheaves are used to calculate concrete cohomology groups -- see injective sheaves.

Applications

Subsequently there were further technical extensions (for example in Godement's book), and areas of application. For example, sheaves were applied to transformation groups; as an inspiration to homology theory in the form of Borel-Moore homology for locally compact spaces; to representation theory in the Borel-Bott-Weil theorem; as well as becoming standard in algebraic geometry and complex manifolds.

The particular needs of étale cohomology were more about reinterpreting sheaf in sheaf cohomology, than cohomology, given that the derived functor approach applied. Flat cohomology, crystalline cohomology and successors are also applications of the basic model.

Euler characteristics

The Euler characteristic $\chi (\mathcal{ℱ})$ of a sheaf $\mathcal{ℱ}$ is defined by

$\chi (\mathcal{ℱ}):=\sum _{i\in {\mathbf{Z}}_0^{+}}(-1{)}^i\mathrm{r}\mathrm{a}\mathrm{n}\mathrm{k}(H^i(X,\mathcal{ℱ})).$

To make sense of this expression, which generalises the Euler characteristic as alternating sum of Betti numbers, two conditions must be fulfilled. Firstly the summands must be almost all zero, i.e. zero for $i\ge N$ for some $N$. Further, rank must be some well-defined function from module theory, such as rank of an abelian group or vector space dimension, that yields finite values on the cohomology groups in question. Therefore finiteness theorems of two kinds are required.

In theories such as coherent cohomology, where such theorems exist, the value of χ(F) is typically easier to compute, from other considerations (for example the Hirzebruch-Riemann-Roch theorem or Grothendieck-Riemann-Roch theorem), than the individual ranks separately. In practice it is often H⁰(X,F) that is of most interest; one way to compute its rank is then by means of a vanishing theorem on the other Hⁱ(X,F). This is a standard indirect method of sheaf theory to produce numerical results.

Relationship with singular cohomology

For a locally contractible topological space, the singular cohomology groups with coefficients in A agree with the sheaf cohomology groups with the constant sheaf of A, for any abelian group A.

References

Almost any reference on sheaves treats sheaf cohomology, for example:

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• Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
  
  Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
  
  In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
  
  Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
  
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  15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
  
  To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010, in the topological setting
• The thread "Sheaf cohomology and injective resolutions" on MathOverflow