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Remke Kloosterman's user avatar
Remke Kloosterman's user avatar
Remke Kloosterman's user avatar
Remke Kloosterman
  • Member for 14 years, 4 months
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Explicit examples of resolution of (projective) 3-folds over k?
You can use the computer algebra package "Singular" to create a bunch of examples of your own. The resolution of ADE-singularities (the easiest examples) in dimension 3 are essentially the same as in dimension 2 and are probably not treated for that reason. To work out the resolution of an A_k-singularity (x^2+y^2+z^2+w^(k+1)) is a nice exercise. Several people tried to convince me that starting from dimension 3 in many applications (e.g. calculation of the hodge structure on the cohomology) it might be easier to work with the singular variety itself rather than pass to the resolution.
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Calculations of Pic^0, Pic, NS of surfaces
In the above paper the Picard lattice of a very general member of the family is computed. Although this is highly non-trivial, it is definitely easier then calculating the Picard lattice of a specific member in the family. (The OP asked for specific examples.) I.e., for a very general surface in $\mathbb{P}^3$ of degree at least 4 one has $Pic(X)=NS(X)=\mathbb{Z}$ (theorem of Noether and Lefschetz); for a concrete surface in $\mathbb{P}^3$ the calculation of $Pic(X)$ is more complicated.
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Degree of canonical bundle?
In general $omega_X$ is not the restriction of a line bundle on $\mathbb{P}^n$. An example is the twisted cubic curve in $\mathbb{P}^3$. In this case the canonical bundle has degree -2, whereas every line bundle obtained by restriction has degree divisible by 3. You can have a look on the final section of chapter IV of Hartshorne. He gives a discussion which pairs (d(C),g(C)) are possible for smooth space curves C. In particular, it is shown that for fixed d there are many possibilities for the genus of C. (Provided that d is not 1 or 2.)
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How do you prove that a field is isomorphic to C(x)?
Maybe you could give a hint what you know about K, (since you claim you do not have a presentation in terms of generators and relations).
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