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

5 votes
2 answers
1k views

Generalize the Proj construction?

I'm wondering if there is a generalization of the Proj construction used in algebraic geometry. Given a graded ring R, which is a monoid homomorphism $R\to \mathbb{N}$, we can form the scheme Proj(R), …
36min's user avatar
  • 3,806
9 votes
1 answer
722 views

Use of Hilbert Schemes in Arithmetic?

I'm curious about the following: What are some arithmetic application of the Hilbert Schemes? The application of Hilbert schemes in algebraic geometry seems to be a great success, from birational geo …
36min's user avatar
  • 3,806
2 votes
0 answers
106 views

What does the term "3-fold vertex" mean in enumerative geometry?

I read about enumerative geometry recently, namely something about the Gromov-Witten, Donaldson-Thomas and Pandharipande-Thomas invariants, and I was trying to see the picture. It seems like the term …
36min's user avatar
  • 3,806
20 votes
2 answers
3k views

Can Chern class/character be categorified?

The Chern character sends the class of a locally free sheaf to the cohomology ring of the underlying variety X. And it is a ring homomorphism from K to H^*. I saw people write its source as the bounde …
36min's user avatar
  • 3,806
10 votes
3 answers
2k views

Families of ideal sheaves: What's the correct definition?

I'm looking at Bridgeland's paper "Flops and Derived categories" and I got confused on what he meant by a family of ideal sheaves. Let $Y$ be a scheme, and let $S$ be another scheme. A family of sheav …
36min's user avatar
  • 3,806
13 votes
2 answers
2k views

Are non-algebraic stacks useful in algebraic geometry?

The title is a bit vague. What I want to know is if there is any geometric application of non-algebraic stacks. I know e.g. the category of coherent sheaves is an example. But I want to ask if people …
36min's user avatar
  • 3,806
15 votes
2 answers
2k views

How to compute the Picard rank of a K3 surface?

I'm curious about the following question: Given a K3 surface, how does one proceed to compute its rank? Of course the answer may depend on the form of the input, i.e. how the K3 is "given". So …
36min's user avatar
  • 3,806
7 votes

Interpreting $f^*f_*$

To me, pushforword is like taking sections along the fibers, and higher pushforwards are like cohomologies along the fiber. Think about that $f^*f_*F\to F$ being surjective as globally generated alon …
36min's user avatar
  • 3,806