Questions tagged [class-field-theory]

The tag has no usage guidance.

Filter by
Sorted by
Tagged with
4 votes
1 answer
255 views

Kummer congruences for totally real number fields

There is a generalization of the Kummer congruences to totally real number fields with characters due to Deligne-Ribet. For example, see the exposition here, more precisely see Theorem 2.1. What is ...
Asvin's user avatar
  • 7,648
2 votes
1 answer
174 views

$[J_F : P_FN_{E/F}J_E] = 2$ for quadratic extensions of global fields of characteristic 2?

Let $E/F$ be a quadratic extension of global fields. Denote by $J_F$ the idèle group of $F$, by $P_F$ the subgroup of principal idèles and by $N_{E/F} : J_E \to J_F$ the norm map between idèles. In ...
Bib-lost's user avatar
  • 267
3 votes
0 answers
226 views

Reference for: power residue symbols are Hecke characters

Notation. Let $n$ be a positive integer, let $\mu_n\subseteq \mathbb C$ be the set of $N$-th roots of unity, let $K$ be a number field containing $\mu_n$, let $R$ be the ring of integers of $K$, let $...
Luca Ghidelli's user avatar
4 votes
0 answers
189 views

Restricted Iwasawa theory

Let $p$ be a prime number, let $K$ be a number field, and let $L$ be a Galois extension of $K$ such that the Galois group $\Gamma$ of $L$ over $K$ is (continuously) isomorphic to the (additive) group $...
Pablo's user avatar
  • 11.2k
6 votes
0 answers
681 views

What are the fastest ways to calculate class number of number fields?

Given a number field $K$, which approaches help us to calculate the class number $h(K)$ of $K$? I am aware that the question is broad but any argument would be helpful. Some basic approaches I know:...
Ninja's user avatar
  • 161
5 votes
1 answer
169 views

Is there a field with finitely many abelian extensions, that is neither separably closed nor real closed?

If $K$ has only finitely many Galois extensions, then $K$ must be either separably closed or real closed. Are there any other fields whose abelianizations are finite extensions (i.e. whose absolute ...
Alex Mennen's user avatar
  • 2,090
6 votes
2 answers
308 views

Computing the relative class group (with Galois action) of relatively large cyclotomic groups

For a cyclotomic field $K = \mathbb Q(\zeta_n)$, let $K^+$ be its maximal totally real subfield. We know that $H^+ = Cl(K^+)$ injects into $H = Cl(K)$. I am interested in computing the group $H/H^+$ ...
Asvin's user avatar
  • 7,648
10 votes
1 answer
665 views

Tables of class numbers of cyclotomic fields

Does anyone have a table of the class numbers ($h_n$) of cyclotomic fields (upto say, n = 250-300 for $\mathbb Q(\mu_n)$)? I can find tables for the relative class number ($h_n^-$) in various places ...
Asvin's user avatar
  • 7,648
6 votes
1 answer
391 views

Unramified non-abelian extension and Galois cohomology

Is there an example of a finite Galois extension $E/F$ of number fields, such that $G=\mathrm{Gal}(E/F)$ is non-abelian and the order of the cohomology group $H^1(G,U_E)$ is relatively prime to class ...
A. Maarefparvar's user avatar
10 votes
0 answers
578 views

A formal group scheme in explicit local class field theory

Let $K$ be a nonarchimedean local field with residue field $k$ of characteristic $q = p^N$, and pick a uniformizer $\pi\in \mathscr{O}_K$. Recall that explicit local class field theory, à la Lubin--...
skd's user avatar
  • 5,550
1 vote
1 answer
322 views

Brauer group of global fields

Is the Brauer group $\text{Br}(K)$ of a global field $K$ an $\ell$-divisible group for some prime $\ell$? If so, what $\ell$? Is $\text{Br}(K)[n]$ finite, for $n$ integer? I know from class field ...
user avatar
1 vote
0 answers
91 views

Abelian group extensions

Let $K$ be an imaginary quadratic field and $E$ be an elliptic curve with CM by $\mathcal{O}_K$. Is there a way to see that $K(j(E), h(E[\mathfrak{p}]))/K$ is an Abelian extension for some $\mathfrak{...
debanjana's user avatar
  • 1,161
10 votes
1 answer
1k views

What are the primes that are ramified?

Let $K$ be an imaginary quadratic field and $E$ be an elliptic curve with CM by $\mathcal{O}_K$. We know that the maximal unramified extension (Hilbert class field) $H/K$ is $K(j(E))$. Can we ...
debanjana's user avatar
  • 1,161
3 votes
1 answer
567 views

The Genus field and Hilbert class field

Is there an example of a number field $K$ for which the genus field of $K$ is contained strictly in the Hilbert class field of $K$?
A. Maarefparvar's user avatar
2 votes
0 answers
224 views

Global sections of higher direct images

If $f : X\to V$ is a smooth proper map of smooth schemes, what are the global sections of $R^if_{fppf, *}\mu_p$ $R^if_{fppf, *}\mathbb{G}_{\rm m}$ I was reading Milne's book "Arithmetic duality", ...
user avatar
2 votes
0 answers
99 views

Fibers of reciprocity maps and higher dimensional analogs

Part I. Say $K$ is a number field, $v$ is a finite place of $K$, $K_v$ the $v$-adic completion of $K$. We have the local Artin map for every finite $v$: $$\rho_v : K_v^{\times}\to\text{Gal}(K_v^{\...
user avatar
7 votes
0 answers
461 views

Explicit $H^2(K, \mu) = Q/Z$?

In the development of local class field theory, a very fundamental theorem is that, for every local field $K$ of characteristic zero, $H^2(K, \mu) \cong \mathbb{Q}/\mathbb{Z}$. $(*)$ Neukirch et al. ...
Evan O'Dorney's user avatar
3 votes
0 answers
109 views

Uniqueness of class field theory map

Let $F$ be a local field of characteristic 0. The main theorems in local class field theory can be summarized by the existence of a group $W_F$ and a map $$ \phi_F:W_F\to W_F^\mathrm{ab}\simeq F^\...
Mike B's user avatar
  • 341
4 votes
1 answer
360 views

Examples of norm forms where the numbers represented can be readily described

In case of impatience: the question here is a request for examples, especially degree six or seven where the norm form might represent some prime$p,$ then some $q^2$ but not $q,$ then some $r^3$ but ...
Will Jagy's user avatar
  • 25.3k
8 votes
1 answer
240 views

Does Ribet's construction of class fields give us eigenspaces of rank 1?

Ribet's paper on the Herbrand-Ribet theorem constructs a representation $\rho: Gal(\overline{\Bbb Q}/\Bbb Q) \to GL_2(\mathbb F_q)$ where $q = p^r$ of the specific form: $ \begin{bmatrix} 1 & *\\ ...
Asvin's user avatar
  • 7,648
3 votes
1 answer
305 views

Inverse image of norm map on principal units for an unramified extension

For a local field $E$, denote by $U(E)$ the units of the corresponding valuation ring $\mathcal{O}_E$, and denote by $U_n(E)$ the prinicipal $n$-units, i.e. $U_n(E)=1+M_E^n$ where $M_E$ is the maximal ...
Gal Porat's user avatar
  • 225
2 votes
1 answer
247 views

correspondence between finite abelian extensions and congruence subgroups

I make self-study in class field theory and I want to prove the following popular fact: Given a modulus $\mathfrak{m}$ of a number field $K$, the map $L\mapsto ker (\phi_{L/K,\mathfrak{m}}$) is an ...
user110832's user avatar
5 votes
2 answers
611 views

The $\ell$- part of the class groups of the $p$-cyclotomic fields

Let $K_n = \Bbb Q(\mu_{p^{n+1}})$ and let $A_n$ be it's class group. Iwasawa theory tells us a lot about the $p$-part of $A_n$. For instance, we know quite a lot about how it varies with $n$. I am ...
Asvin's user avatar
  • 7,648
6 votes
1 answer
529 views

Analogue of j-invariant for CM fields

For any imaginary quadratic field $F$, the Hilbert class field $H$ is generated by the $j$-invariant of any elliptic curve with complex multiplication (CM) by $\mathcal O$, the ring of algebraic ...
guest's user avatar
  • 528
1 vote
1 answer
175 views

Hilbert symbols vanishing

Let $p$ be an odd prime, and let $E/\mathbb{Q}_p$ be a finite extension that contains a primitive $p$-th root of unity $\zeta_p$ but not a primitve $p^2$-th root of unity $\zeta_{p^2}$. Let $a,b \in E^...
Pablo's user avatar
  • 11.2k
8 votes
1 answer
751 views

How to compute with the Stark conjectures?

I would like a convenient basis for the elements of a fixed abelian extension $E$ of a real quadratic field $\mathbb{Q}(\sqrt{d})$. The accepted answer to this MO question suggests that the Stark ...
Dustin G. Mixon's user avatar
2 votes
1 answer
415 views

Complete fields with algebraically closed residue field

I am looking for a reference where the following result is proven: Let $k$ be an algebraically closed field. If $K$ is a complete and discretely valued field with residue field $k$. Then $K$ is one ...
user223794's user avatar
2 votes
1 answer
318 views

CM Elliptic Curves and a result concerning ray class fields

Let $K$ be an imaginary quadratic field and suppose that $E/K$ has complex multiplication by $\mathcal{O}_K$. Let $\psi$ be the Hecke character associated with $E$ and $\mathfrak{f}$ its conductor (i....
Alexandre Daoud's user avatar
5 votes
1 answer
457 views

Artin map restricted to base field

Let $M/L/K$ be a tower of local fields such that $M/L$ is abelian with Galois group $G$. The Artin map $\psi_{M/L}$ restricted to $K^\times$ is a continuous map to $G$ and thus corresponds to some ...
Evan O'Dorney's user avatar
4 votes
0 answers
243 views

Polynomial equations in many variables have solutions (Lang 1952 paper)

I am trying to understand the proof of the following result: Suppose $F$ is a function field in $k$ variables over an algebraically closed field. Let $f_1,...,f_r \in F[x_1,...,x_n]$ be ...
user223794's user avatar
2 votes
0 answers
98 views

Quasi-algebraically closed fields reference request

I am looking for a road to understanding what quasi-algebraically closed fields are with the ultimate goal of understanding the paper by Lang 1952. My current background is the first 6 chapters from ...
user223794's user avatar
4 votes
1 answer
550 views

Hilbert Symbols, Norms, and p-adic roots of unity

Let $p$ be an odd prime number, let $\mathbb{Q}_p$ be the field of $p$-adic numbers, and let $\overline{\mathbb{Q}_p}$ be an algebraic closure of it. For a primitive $p$-th root of unity $\zeta_p \in ...
Pablo's user avatar
  • 11.2k
3 votes
1 answer
611 views

Determining the image of the norm map of a cyclic extension

Suppose $L / K$ is a cyclic extension of number fields. Is there a straightforward way to determine if a given $\alpha\in K^{*}$ is in the image of the norm map $N_{L/K}:L^{*}\rightarrow K^{*}$? Or to ...
Matt Westaway's user avatar
2 votes
0 answers
550 views

Valuation topology vs modified valuation topology

Let $K$ be a field with valuation $v:K\to G\cup\{\infty\}$ where $G$ is an ordered abelian group. In section 7.62 of the book "Foundations of analysis over surreal number fields." Vol. 141. Elsevier, ...
Chilote's user avatar
  • 586
5 votes
1 answer
885 views

p-adic expansion for elements in algebraic closure of p-adic numbers

In the following I will describe a proposal for the p-adic expansion of the elements of the algebraic closure $\overline{\mathbb{Q}_p}$ of $\mathbb{Q}_p$. My question is if this "conjecture" has been ...
Chilote's user avatar
  • 586
5 votes
1 answer
345 views

Ray class groups through binary quadratic forms

(Cross-posted from https://math.stackexchange.com/questions/2029407/ray-class-groups-through-binary-quadratic-forms) If $d$ is the discriminant of a quadratic number field, then the primitive classes ...
Barry's user avatar
  • 1,501
47 votes
1 answer
3k views

A three-line proof of global class field theory?

There is an idea (I think originally due to Tate) that class field theory is fundamentally a consequence of Pontrjagin duality and Hilbert Theorem 90. I'm curious whether this can phrased using modern ...
Dmitry Vaintrob's user avatar
5 votes
0 answers
201 views

Real field of definition of an abelian variety of CM-type?

Question 0. Can a field of definitions (without automorphisms) of an (almost arbitrary) abelian variety of CM-type, originally defined over ${\mathbb{C}}$, be chosen to be a totally real number ...
Mikhail Borovoi's user avatar
23 votes
1 answer
2k views

Any open Langlands Conjectures for GL_1?

Are there any general conjectures/properties (in the Langlands Program) for automorphic representations of $GL_n$ which are still open for $n=1$?
Eins Null's user avatar
  • 1,579
7 votes
4 answers
597 views

Is cohomology of groups all about $H^{i}: -2\leq i\leq 2$?

I am reading Class field theory - Bonn Lectures by Neukirch. Given a $G$ module $A$ he defines Cohomology groups $H^i(G,A) : i\in \mathbb{Z}$ by considering some complete resolution of $G$ modules ...
user avatar
7 votes
1 answer
383 views

Existence of imaginary quadratic fields of class numbers coprime to $p$ with prescribed splitting behaviour of $p$

Let $x\in\{\text{totally ramified, inert, totally split}\}.$ If $p\geq 5$ is a prime, are there infinitely many imaginary quadratic fields $K=\mathbb{Q}(\sqrt{-d})$ of class number coprime to $p$ so ...
The Thin Whistler's user avatar
7 votes
1 answer
750 views

Does complex multiplication for higher dimensional abelian varieties give some generalization of class field theory?

I am currently learning some aspects of the theory of complex multiplication for elliptic curves, and the relationship with class field theory. As I understand it, there is a very special class of ...
dorebell's user avatar
  • 2,968
2 votes
0 answers
86 views

Rank growth in ray class fields of primes that are inert in an imaginary quadratic extension

If $K=\mathbb{Q}[\sqrt{-d}]$, $E$ is an elliptic curve and $\operatorname{rank}(E(K))=\operatorname{rank}(E(\mathbb{Q}))$, are there infinitely many primes $l$ of $\mathbb{Q}$ so that: (1.) $\...
The Thin Whistler's user avatar
25 votes
1 answer
3k views

On the history of the Artin Reciprocity Law

At the beginning of Milne's notes on class field theory, he has a quote by Emil Artin (as recalled by Mattuck in Recountings: Conversations with MIT mathematicians): I will tell you a story about ...
Asvin's user avatar
  • 7,648
2 votes
0 answers
163 views

Relative Leopoldt defect

Let F be a totally real number field such that the Leopoldt conjecture holds at a prime number $p$ and $M$ be a quadratic totally real extension of $F$. Is there a bound of the Leopoldt defect of $M$ ...
Adel BETINA's user avatar
  • 1,046
9 votes
1 answer
521 views

Globalizing local field extensions with controlled ramification

Let $K_1/k_1, \ldots, K_r/k_r$ be "separable cyclic" extensions of degree $n$ where each $k_i$ is a local field of characteristic 0 (archimedean or not). By separable cyclic of degree $n$, I mean the ...
Kimball's user avatar
  • 5,709
3 votes
1 answer
316 views

Connected-étale sequence for ordinary CM elliptic curves

Let $E/k$ be an elliptic curve over algebraically closed field of characteristic $p$ with CM, for simplicity, by the maximal order of a quadratic imaginary field $K/\mathbb{Q}$. Suppose that $p$ is ...
Bear's user avatar
  • 845
1 vote
0 answers
145 views

Class field theory, Ideles class

Let $H$ be a totally complex Galois extension of $\mathbb{Q}$ and $g:G_H \rightarrow \bar{\mathbb{Q}}_p$ be a continuous morphism. By class field thoery we have $\mathrm{Hom}(G_H, \bar{\mathbb{Q}}_p)\...
Adel BETINA's user avatar
  • 1,046
2 votes
0 answers
127 views

The Galois side of the Norm map

Let $K$ be an abelian extension of $\mathbb{Q}$. We know that $[x, K]|_{\mathrm{Gal}{\mathbb{Q}^{ab}}}=[\mathrm{N}^{K}_{\mathbb{Q}} x, \mathbb{Q}]$ where $[x, F]$ is the Artin reciprocity map. Given a ...
Watson Ladd's user avatar
  • 2,419
4 votes
0 answers
296 views

Complex multiplication and ray class fields

This question is mainly referring to the proof of Theorem 5.6, Chapter 2 of Silverman's "Advanced Topics in the AEC". Basically, let $K$ be an imaginary quadratic field, and $E$ be an elliptic curve ...
ReHsu's user avatar
  • 41

1 2 3
4
5
8