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

5 votes

Applications of algebraic geometry over a field with one element

I'm not aware of such a theorem, but you should take a look into absolute zeta-functions, which hope to prove a lot about zeta-functions — perhaps up to Riemann's Hypothesis. Since I don't know much …
Ilya Nikokoshev's user avatar
1 vote

Homology of algebraic varieties in Okounkov's paper on enumerating algebraic curves

It appears that you want the "rough, introductory answers", so feel fine to ask more detailed questions if anything is unclear/potentially wrong. Also, I only have very general knowledge about enumera …
Ilya Nikokoshev's user avatar
3 votes

Looking for an introductory textbook on algebraic geometry for an undergraduate lecture course

You can also take a look at the question A learning roadmap for algebraic geometry.
3 votes

A specific branched cover of S^2 as a subgroup of Pi_1

In standard topological terms, the exact sequence that relates homotopy groups of the base $B$, fiber $F$ and total space $E$ of topological fibration gives $$\pi_1(F) \to \pi_1(E) \to \pi_1(B) \to \ …
Ilya Nikokoshev's user avatar
3 votes

Degree 2 branched map from the torus to the sphere

You can do a reverse construction: start with a sphere without 4 points; now add two points over each one in such a way that every time you go around one hole the two points get interchanged. The sam …
Ilya Nikokoshev's user avatar
14 votes
3 answers
1k views

Non-simply-connected smooth proper scheme over Z?

Source This question came up in the discussion between Kevin Buzzard and Minhyong Kim in the comments to Smooth proper scheme over Z. It was 2 weeks ago, so I took the liberty of posting it as commun …
4 votes
Accepted

A hands-on description of a "completion" of the free commutative monoid on countably many ge...

Preliminaries First part of your question doesn't use the bialgebra structure. That is, you have a space of functions on countable many points which I'll denote $A = \mathbb C_1\times \mathbb C_2\tim …
3 votes

What are the points of Spec(Vassiliev Invariants)?

I don't have much knowledge in either knots or Hopf bialgebras, but I think one can answer part of your question (the one formerly asked as a title: "What do negative knots look like?") anyway. Abou …
Ilya Nikokoshev's user avatar
-1 votes

Blow up along codimension one closed subscheme

How do you define blow-up? It should be straightforward to show that an explicit construction has relative dimension 0 over $X$. Update: In the comments I suggest to take the formal two-dimensional …
Ilya Nikokoshev's user avatar
10 votes

Why no abelian varieties over Z?

Comments by Anweshi The essential point is what Emerton mentioned, ie the analogy with Minkowski's theorem on number fields with ramification. The basic principle is that "arithmetic is geometry". Nu …
33 votes
5 answers
8k views

Why no abelian varieties over Z?

Motivation I learned about this question from a wonderful article Rational points on curves by Henri Darmon. He gives a list of statements (some are theorems, some conjectures) of the form the set $\ …
Ilya Nikokoshev's user avatar
4 votes
Accepted

Field of Definition of a Meromorphic Function

It is not sufficient that the subscheme of poles and zeroes is defined together over $K$, as the example of the function $(z+i)/(z-i):\mathbb P^1 \to \mathbb P^1$, defined only over $\mathbb Q(i)$, il …
Ilya Nikokoshev's user avatar
11 votes

Euler-Maclaurin formula and Riemann-Roch

Yes, this is a big area of research. I'll add some references to the ones Dmitri provides. Here are references from a question about Moment map for toric actions: Riemann-Roch for toric orbifolds b …
Ilya Nikokoshev's user avatar
3 votes
2 answers
390 views

Homology class orthogonal to image of Chern characters?

I had this simple question when formulating the Todd class question. Does there exist an example of proper morphism $f:X\to Y$ together with nontrivial homology class $t\in H^*(X)$ such that for a …
Ilya Nikokoshev's user avatar
60 votes
4 answers
6k views

Why do Todd classes appear in Grothendieck-Riemann-Roch formula?

Suppose for some reason one would be expecting a formula of the kind $$\mathop{\text{ch}}(f_!\mathcal F)\ =\ f_*(\mathop{\text{ch}}(\mathcal F)\cdot t_f)$$ valid in $H^*(Y)$ where $f:X\to Y$ is a …
Ilya Nikokoshev's user avatar

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