31
votes
Accepted
Embedding abelian varieties into projective spaces of small dimension
Recall that any smooth projective variety of dimension $g$ embeds into $\mathbf{P}^{2g+1}$. Consider now an abelian variety $A$ of dimension $g$ which embeds into $\mathbf{P}^{2g}$. Van de Ven proves (...
- 2,808
23
votes
Accepted
When $k = \mathbb{F}_q$ finite field, $X$ always has $k$-rational point, and so $A \simeq X$?
This is a theorem of Lang's from 1956. Here's an online document giving a proof (in the form $H^1(A,k)=0$):
Lecture 14: Galois Cohomology of Abelian Varieties over Finite Fields,
William Stein. http:...
- 47.4k
23
votes
Accepted
On Tate's "Endomorphisms of Abelian Varieties over Finite Fields", sketch of proof of main result?
I don't have any contribution for the intuition beyond the fact that, I can't construct something outside the image of (1) so I hope it's surjective.
Here is a sketch of the central idea of Tate's ...
- 30.5k
18
votes
Is every abelian variety a subvariety of a Jacobian?
Embed the dual abelian variety into projective space. Take a smooth hyperplane section and interate until it's one-dimensional, obtaining a smooth curve $C$. By Lefschetz $C$ is irreducible, and the ...
- 148k
18
votes
Accepted
Is the complement of an affine open in an abelian variety ample?
Welcome new contributor. Yes, that is true. Let $k$ be any field, let $A$ be an Abelian variety over $k$, and let $U\subset A$ be a dense open affine. Denote by $D\subset A$ the complementary ...
16
votes
Accepted
Tate-Shafarevich group over number fields
No.
It is always difficult to "prove" that something is "not known", but this may do: I claim it is not even known when $K=\mathbb Q$, $A$ is an elliptic curve $E$. In fact in this case, the result ...
- 26k
16
votes
Accepted
Points of abelian varieties over purely transcendental extensions
This follows from the following well-known lemma.
Lemma. Let $A$ be an abelian variety over $k$. Then any map $f \colon \mathbb P^1 \to A$ is constant.
Proof 1. The map $f$ induces a map on the ...
- 28.7k
15
votes
Accepted
Canonical lift of the deformation of an ordinary abelian variety
No. The picture over a general base is this: let $A_0\to S_0$ be an ordinary abelian variety over a characteristic $p$ scheme $S_0$, and let $S_n$ ($n\geq 0$) be compatible flat liftings of $S_0$ over ...
- 16.1k
15
votes
Accepted
Torsion points of abelian variety as zeros of a section of a vector bundle?
The crucial case is $m=1$: if you have a vector bundle $E$ on $A$ of rank $\dim(A)$ and a section $s$ of $E$ whose zero locus is $\{0\} $, pulling back $(E,s)$ by multiplication by $m$ gives the ...
- 38k
14
votes
When $k = \mathbb{F}_q$ finite field, $X$ always has $k$-rational point, and so $A \simeq X$?
A bit of overkill, but it follows from the Weil conjectures. The structure of cohomology ($H^i = \wedge^i H^1$) is computed over the algebraic closure and it follows that the number of points is $\...
- 30.5k
14
votes
Accepted
Albanese variety over non-perfect fields
The arguments of Serre can be in fact made to work over any separably closed field. The result in the general case can then be deduced using Galois descent. Details can be found in Section 2 and the ...
- 21.1k
14
votes
Albanese variety over non-perfect fields
The answer is affirmative (in Serre's formulation via principal homogeneous spaces) for proper, geometrically reduced, and geometrically connected schemes $X$ over any field $k$, giving a strong ...
14
votes
Accepted
On the moduli stack of abelian varieties without polarization
First, when defining the stack you will have the issue that there are formal deformations of abelian varieties which do not extend to families of abelian varieties over any reduced scheme. These are ...
- 148k
14
votes
Accepted
Is a complex algebraic set with a Zariski dense subset of algebraic points already defined over the algebraic numbers?
More is true. Let $K/F$ be a field extension. Let $X$ be the vanishing set of some polynomials in $K[X_1,\dots, X_n]$. If $X$ contains a Zariski dense set of points with coordinates in $F$, then $X$ ...
- 148k
13
votes
Bhargava's work on the BSD conjecture
For a prime $p$ and an elliptic curve $E/\mathbb{Q}$, we have the exact sequence
$$\displaystyle 0 \rightarrow E(\mathbb{Q})/p E(\mathbb{Q}) \rightarrow S_p(E) \rightarrow \text{Sha}_E[p]\rightarrow ...
- 26.9k
13
votes
Is every abelian variety a subvariety of a Jacobian?
Let me give an answer for $k = \mathbb{C}$.
By a theorem of Matsusaka, every abelian variety $A$ over an algebraic closed field $k$ is a quotient of a Jacobian.
Now just apply Matsusaka's theorem to ...
- 66.3k
13
votes
Accepted
Conceptual explanations of the class numbers for the first few $\mathbb{Q}(\sqrt{p})$ with odd conductor
We give a uniform approach to $p \leq 61$ by applying
analytic discriminant bounds to the Hilbert class field.
To be sure this is not entirely "conceptual", but then
some computation is needed even to ...
- 79.9k
12
votes
Accepted
Galois action on $p$-adic Tate module of Abelian variety over finite field semisimple?
You're just asking whether Frobenius acts semsimply on the $p$-adic Tate module.
We know from Tate's theorem that Frobenius acts semisimply on the $\ell$-adic Tate module, and hence satisfies some ...
- 148k
12
votes
Accepted
Are there infinitely many real multiplication fields of abelian surfaces over $\mathbb Q$?
A conjecture of Coleman asserts that only finitely many rings arise as the endomorphism ring of an abelian variety of given dimension defined over a number field of given degree. See [1] for an ...
- 20.8k
11
votes
Accepted
Do all simple factors of jacobians of curves come from correspondences?
This argument is mostly contained in t3suji's comments, but with some of the proofs somewhat expanded.
As a convention (consistent with that of the theory of Chow motives), all actions of ...
- 28.7k
11
votes
Torsion points of abelian varieties in the perfect closure of a function field
The answer to question (*) is yes. It is Theorem 1.2.2 in the following preprint.
- 728
11
votes
Accepted
Is the symmetric product of an abelian variety a CY variety?
When $\dim A = 1$, $S^nA$ is a $\mathbb{P}^{n-1}$-bundle over $A$, so its Kodaira dimension is $-\infty$.
When $\dim A = 2$, the minimal resolution of $S^nA$ is given by the Hilbert scheme $A^{[n]}$, ...
- 39.3k
11
votes
Accepted
Tamagawa numbers
Denote by $\Phi$ the quotient of $\mathcal A^\vee$, the special fiber of the smooth (but not necessarily proper) model of the dual abelian variety $A^\vee$, by the connected component of $0$ of $\...
- 10.9k
11
votes
Accepted
Which schemes are divisors of an abelian variety?
Any curve of genus greater than two, whose Jacobian $J$ is simple, will do. If it were a divisor on an abelian surface $S$, then there would be a surjection $J\to S$ with positive dimensional kernel, ...
- 2,606
11
votes
A geometric definition of the addition law on abelian surfaces
This must be standard, I don't have a reference but the construction is easy:
let $y^2=f(x)$ be a genus 2 hyperelliptic curve with $f$ squarefree of degree
$5$ or $6$. As a set the Jacobian is the ...
- 13.1k
11
votes
Accepted
Do there exist elliptic curves over $H_K$ having everywhere good reduction and CM by $\mathcal{O}_K$?
Here is an example.
Let $K = \mathbf{Q}(\sqrt{-21})$. Then the class group of $K$ is $C_2 \times C_2$ and its Hilbert class field is $H_K = \mathbf{Q}(\sqrt{-1}, \sqrt{3}, \sqrt{7})$. In particular, $...
- 37k
11
votes
Accepted
A "comprehensive" family of abelian varieties
Welcome new contributor. The idea of such "comprehensive" families goes back very far. These were studied by Amitsur under the name "generic splitting varieties", primarily in ...
10
votes
Accepted
How to compute the formal group law of a Shimura variety (using its invariant differentials)?
I don't know how to do this purely with invariant differentials. One
of the main reasons is that, in order to solve this problem, you need
to construct an "algebraic" lift of this variety from the ...
- 52.6k
10
votes
Geometry of Albanese image
The first natural observation is the following. Set as usual $q(X):=h^{1, \,0}(X)$, so that $q(X)= \dim \mathrm{Alb}(X)$. Then the Albanese map of $X$ cannot be surjective if $\dim X < q(X)$, and ...
- 66.3k
10
votes
Accepted
Can an abelian variety dominate a variety of general type?
Theorem: Let $A$ be an abelian variety and $f:A\to X$ a dominant morphism to a projective variety of general type, then $\dim X=0$.
Proof: Replacing $f$ by a birational model of the Stein ...
- 2,427
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