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This tag is used if a reference is needed in a paper or textbook on a specific result.

4 votes
0 answers
123 views

Unramified Galois cohomology

Let $k$ be a local field with absolute Galois group $\Gamma_k$, inertia subgroup $I_k \subset \Gamma_k$, and residue field $\mathbb{F}$. Let $M$ be a finite Galois module over $k$. The unramified Galo …
Daniel Loughran's user avatar
11 votes
0 answers
364 views

Example of abelian variety over finite field which doesn't lift

What is an example of an abelian variety over a finite field $\mathbb{F}_p$ which doesn't lift to $\mathbb{Z}_p$? This question seems to hint that they should exist, but no example is given. Note that …
Daniel Loughran's user avatar
2 votes

Naive question on the Jacobian of a curve

If the Neron-Severi group of an abelian variety is $\mathbb{Z}$, then a principal polarisation, if it exists, is unique. This is the generic case, as well as for generic jacobians. To find counter-exa …
Daniel Loughran's user avatar
4 votes
Accepted

Hardy-Littlewood circle method for non-diagonal quadratic forms

The "best" way to deal with quadratic forms using the circle method is via Heath-Brown's delta symbol method. You can read about this in detail in the paper: Heath-Brown - A New Form of the Circle Met …
Daniel Loughran's user avatar
4 votes

Character sums concerning $a^x-1$

Shparlinski has done a lot of work on problems like this, in the more general setting of linear recurrence sequences. (Your sequence of Mersenne numbers is such a sequence). I'd recommend looking at C …
Daniel Loughran's user avatar
14 votes

Building algebraic geometry without prime ideals

This nice approach to points on schemes in fact becomes crucial once one leaves the world of schemes and travels to the galaxy of stacks. For an algebraic stack $X$, one defines a point of $X$ to be a …
Daniel Loughran's user avatar
11 votes
Accepted

Reference request: Long exact sequence in profinite Galois cohomology up through $H^2$

See Section 5.7 of Serre's Galois Cohomology. (In general Chapter 5 of this book is a fairly definitive reference for non-abelian cohomology, and he works with an arbitrary profinite group $G$).
Daniel Loughran's user avatar
12 votes
1 answer
521 views

Equidistribution of $\{\alpha p\}$ for $p$ in an arithmetic progression

Let $\alpha$ be irrational. A famous theorem of Vinogradov says that $\{ \alpha p\}$ is equidistributed in $[0,1]$ as $p$ runs over all primes. Let $a,q$ be natural numbers with $\gcd(a,q) = 1$. Then …
Daniel Loughran's user avatar
6 votes

Reference request: Diophantine equations

It is difficult to get far in the modern theory without some algebraic geometry. This is the approach taken in the book: Bjorn Poonen, Rational points on varieties, Graduate Studies in Mathematics 18 …
Daniel Loughran's user avatar
1 vote
Accepted

Existence of analytic continuation of Dirichlet series corresponding to the indicator sequen...

Serre deals with problems of this type in the paper: Serre -Divisibilité de certaines fonctions arithmétiques The fact you want should follow from the results in Sections 1 and 2. Alternatively, th …
Daniel Loughran's user avatar
1 vote
Accepted

Logarithms of $L$-functions of irreducible characters of Galois group

Yes this is the fact that for a non-trivial irreducible Artin character, the associated Artin L-function is holomorphic and non-zero on $\rm{re}\, s \geq 1$. For the trivial character, one just obtai …
Daniel Loughran's user avatar
3 votes

Computing the class group of a quadratic function field

The answer to the precise question is yes. See Theorem 1.2 of: "Homological stability for Hurwitz spaces and the Cohen-Lenstra conjecture over function fields," https://arxiv.org/pdf/0912.0325.pdf. …
Daniel Loughran's user avatar
6 votes
Accepted

Maps from products of Brauer-Severi varieties and sections

This is false. Take $X$ to be a smooth plane conic without a rational point. Consider the surface $$S = X \times X.$$ Over the algebraic closure this becomes isomorphic to $\mathbb{P}^1 \times \mathbb …
Daniel Loughran's user avatar
14 votes
Accepted

Smooth proper variety over $\mathbb Q$ with everywhere bad reduction

As explained in the comments, there is no such variety. This is an application of a general set of techniques called "spreading out". You can find a very nice treatment of this in Chapter 3 of the bo …
Daniel Loughran's user avatar
19 votes
5 answers
2k views

Sum of the reciprocals of radicals

Recall that the radical of an integer $n$ is defined to be $\operatorname{rad}(n) = \prod_{p \mid n } p$. For a paper, I need the result that $$\sum_{n \leq x} \frac{1}{\operatorname{rad}(n)} \ll_\v …
Daniel Loughran's user avatar

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