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Prime numbers, diophantine equations, diophantine approximations, analytic or algebraic number theory, arithmetic geometry, Galois theory, transcendental number theory, continued fractions

29 votes
1 answer
2k views

Is the Brauer group of a surface an elliptic curve?

Of course not. But after reading a bit, some points make me believe it should be: Let $S$ be a nice$^{\*}$ surface defined over $Spec\ \mathbb{Z}$. The Brauer group $Br(S\otimes \bar{\mathbb{Q}})$ …
Dror Speiser's user avatar
  • 4,593
29 votes
2 answers
4k views

What is the algebraic closure of the field with one element?

If doing geometry over $\mathbb F_p$ means also using its algebraic closure, it must be interesting to talk about the algebraic closure of $\mathbb F_1$ - the field with one element. I saw that the f …
Dror Speiser's user avatar
  • 4,593
28 votes
Accepted

Why does the definition of modularity demand weight 2?

$\newcommand\Q{\mathbf{Q}}$ $\newcommand\Qbar{\overline{\Q}}$ $\newcommand\Gal{\mathrm{Gal}}$ $\newcommand\C{\mathbf{C}}$ $\newcommand\Sym{\mathrm{Sym}}$ $\newcommand\E{\mathcal{E}}$ $\newcommand\Bett …
27 votes
5 answers
3k views

Class number measuring the failure of unique factorization

The statement that the class number measures the failure of the ring of integers to be a ufd is very common in books. ufd iff class number is 1. This inspires the following question: Is there a quant …
Dror Speiser's user avatar
  • 4,593
26 votes
7 answers
6k views

When is a product of elliptic curves isogenous to the Jacobian of a hyperelliptic curve?

David's question Families of genus 2 curves with positive rank jacobians reminded me of a question that once very much interested me: when is a product of elliptic curves isogenous to the jacobian of …
Dror Speiser's user avatar
  • 4,593
25 votes
1 answer
1k views

When does a modular form satisfy a differential equation with rational coefficients?

Given a modular form $f$ of weight $k$ for a congruence subgroup $\Gamma$, and a modular function $t$ with $t(i\infty)=0$, we can form a function $F$ such that $F(t(z))=f(z)$ (at least locally), and w …
Dror Speiser's user avatar
  • 4,593
18 votes
1 answer
2k views

Galois representations attached to Maass form

So, how does one construct a galois representation from a Maass form? For a modular cusp eigenform, I am familiar with the work of Eichler-Shimura, Deligne, Deligne-Serre, and realize these are diffe …
Dror Speiser's user avatar
  • 4,593
17 votes
5 answers
3k views

Families of number fields of prime discriminant

When I am testing conjectures I have about number fields, I usually want to control the ramification, especially minimize to a single prime with tame ramification. Hence, I usually look for fields of …
Dror Speiser's user avatar
  • 4,593
16 votes
4 answers
1k views

Geometric meaning of fiber of modular parameterization over a point of an elliptic curve?

Given an elliptic curve $E/\mathbb{Q}$ of conductor $N$, parameterization $\psi : X_0(N) \rightarrow E$, and a point $P \in E$, take the fiber $\psi^{-1}(P)$. Its points, being on $X_0(N)$, correspond …
Dror Speiser's user avatar
  • 4,593
14 votes
2 answers
2k views

Irreducible polynomial over number field with roots in every completion?

Let K/Q be a field, probably not a finite extension. Is it possible for a polynomial to be irreducible over K but have a root in every completion of K? What about all but finitely many completions? T …
Dror Speiser's user avatar
  • 4,593
13 votes
Accepted

Upper bound of period length of continued fraction representation of very composite number s...

The continued fraction length is usually a small constant factor away from the regulator. A more precise version can also be achieved, but I don't remember a reference, so if anyone does... Then, we …
Dror Speiser's user avatar
  • 4,593
12 votes
Accepted

Examples of Galois-invariant central simple algebras which aren't base change?

[big edit] (1) Let $L/K$ be as above. Take any non-identity element $\sigma \in G_{L/K}$, and let $F=L^{<\sigma>}$ be the fixed field of the cyclic subgroup generated by $\sigma$. By your comment abo …
Dror Speiser's user avatar
  • 4,593
12 votes
0 answers
747 views

What is the Shafarevich-Tate group of GL(2)?

Let $K$ be a number field, $\bar{K}$ a fixed algebraic closure, $G_K$ the absolute galois group, $O_\bar{K}$ the ring of integers of the algebraic closure, $\mathfrak{p}$ runs over prime ideals and su …
Dror Speiser's user avatar
  • 4,593
11 votes
1 answer
562 views

CM field to Torus to Abelian Variety?

Given a CM field we can use its maximal order (and a choice of CM type) to construct an abelian variety $\mathbb{C}^g/\Lambda$ with complex multiplication by the maximal order. How do I (or where can …
Dror Speiser's user avatar
  • 4,593
11 votes
2 answers
911 views

Density of monogenic number fields?

Background Zev Chonoles recently asked the question "which number fields are monogenic?". The answers say that for a specific number field the question is hard. So, I thought, how about looking at al …
Dror Speiser's user avatar
  • 4,593

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