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Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.

18 votes

Hironaka desingularisation theorem -- new proofs in literature?

you might be looking for Kollar's book Lectures on Resolution of Singularities
David Lehavi's user avatar
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15 votes
Accepted

When is a scheme a zero-set of a section of a vector bundle?

As for the first question, the class of X has to be the product of the Chern roots of the bundle, so in the Chow ring, it is the class of a complete intersection. As for the second question, you woul …
David Lehavi's user avatar
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13 votes
Accepted

Moduli space of K3 surfaces

I think you are looking for this http://arxiv.org/pdf/math/0506120 which is the same as: Rizov, Jordan Moduli stacks of polarized $K3$ surfaces in mixed characteristic. Serdica Math. J. 32 (2006), …
David Lehavi's user avatar
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12 votes
Accepted

Moduli spaces of complex curves as algebraic varieties

The classical (pre Deligne-Mumford) approach is to map $\mathcal{M}_g$ into $\mathcal{A}_g$ using the Torreli map. Whereas the classical way to see that $\mathcal{A}_g$ is quasi-projective is to defin …
David Lehavi's user avatar
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10 votes

Italian school of algebraic geometry and rigorous proofs

The funniest example I know of is the number of conics tangent to five conics. There are now a number of different proofs, all based on modern intersection theory. Over the years, and until Fulton and …
9 votes
1 answer
1k views

Visualizing a complex plane cubic together with the real plane

In Alain Roberts "Elliptic curves: notes from postgraduate lectures given in Lausanne 1971/72" page 11 (available on google books unless you already tried to read another chapter), there is a hand dra …
David Lehavi's user avatar
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9 votes

Computing fundamental groups and singular cohomology of projective varieties

The trick I know (learned it from Ron Livne) is to project it to some space with known homotopy / homology, throw away the ramification and branch loci to get a covering map (and you better pray it's …
David Lehavi's user avatar
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9 votes

Etale cohomology -- Why study it?

One of the annoying aspects with sheaf cohomology in algebraic geometry is that - even for curves over the complex numbers - the cohomological dimensions are not what you want them to be, if you are u …
David Lehavi's user avatar
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8 votes
Accepted

K3 over fields other than C?

The "standard" definition of a K3 surface is field independent (unless you are a physicist): $p_g=1, q=0$, and trivial canonical class. Some results: Mumford and Bombieri showed that you get (just …
David Lehavi's user avatar
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8 votes

Why is a variety of general type hyperbolic?

you can blow up a point on a general type surface, and get a general type surface containing a copy of C.
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7 votes

The importance of EGA and SGA for "students of today"

Regarding EGA, I think the most appropriate answer is: "wy bother ?". Unless you have a really special interest, you shouldn't. Edit Expanding on this (it seems a lot of people seems it's just flame …
7 votes
3 answers
1k views

How many independent quadrics should one intersect to get the canonical curve.

Let $C$ be a non hyperelliptic complex algebraic curve of genus $g$, then the vector space $I_2(C)$ of quadrics containing the canonical image of $C$ is $\binom{g+1}{2}-h^0(2\omega_C) = (g-2)(g-3)/2$ …
David Lehavi's user avatar
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6 votes

Geometry Vs Arithmetic of schemes

Look at Dan Abramovich's Birational geometry for number theorists
David Lehavi's user avatar
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5 votes

Invariants of higher genus curves

AFAIK, these are know only up to genus 3. Genus 2: Igusa (classical). Hyperelliptic genus 3: Shioda (classical). Non hyperelliptic genus 3: a decade ago by Dixmier & Ohno - see https://www.win.tue.n …
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5 votes

Do there exist modern expositions of Klein's Icosahedron?

There is the outrageously expensive "Geometry of the quintic" by Jerry Shurman, which discusses both Klein and Doyle-McMullen approaches (and then some more).
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