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Algebraic number fields, Algebraic integers, Arithmetic Geometry, Elliptic Curves, Function fields, Local fields, Arithmetic groups, Automorphic forms, zeta functions, $L$-functions, Quadratic forms, Quaternion algebras, Homogenous forms, Class groups, Units, Galois theory, Group cohomology, Étale cohomology, Motives, Class field theory, Iwasawa theory, Modular curves, Shimura varieties, Jacobian varieties, Moduli spaces

1 vote

Non-commuting elements of finite orders in a reductive group over a p-adic field

Here I give details of the reduction in LSpice's comment. I write it as an answer rather than a string of comments in order to have an editable text. The reduction goes as follows. According to Will's …
5 votes
2 answers
175 views

Non-commuting elements of finite orders in a reductive group over a p-adic field

Let $k$ be a $p$-adic field and $G$ be a connected non-abelian reductive algebraic group over $k$. I am asking for a proof of the following lemma: Lemma. Assuming that $p$ is "good" for $G$, there ex …
Mikhail Borovoi's user avatar
1 vote
0 answers
156 views

Computer computation of the first Galois cohomology of a $p$-adic torus?

Let $T\subset {\rm GL}(N,{\mathbb Q})$ be an $n$-dimensional ${\mathbb Q}$-torus given by its Lie algebra $\mathfrak{t}\subset \mathfrak{gl}(N,{\mathbb Q})$. I want to compute, in some sense explicit …
Mikhail Borovoi's user avatar
3 votes
0 answers
107 views

Describing the primes with each cyclic decomposition group in a given finite Galois extensio...

$\newcommand{\Q}{{\mathbb Q}} $Let $f\in \Q[x]$ be a polynomial, and let $L/\Q$ be the finite Galois extension obtaining by adjoining to $\Q$ all roots of $f$. Magma knows how to compute $\Gamma:={\r …
Mikhail Borovoi's user avatar
6 votes
0 answers
167 views

Computer programs for decomposition groups?

There is quite a lot of work on computing Galois groups of splitting fields of polynomials over $\Bbb Q$. Magma is quite good at it. In this answer to Decomposition groups for the Galois module $\mu_8 …
Mikhail Borovoi's user avatar
6 votes

Hilbert's Satz 90 for real simply-connected groups?

In addition to the answer of Gro-Tsen: Let $G=\operatorname{SU}(3)$. Then $H^1({\mathbb R},G)$ classifies matrices of Hermitian forms in 3 variables with determinant 1. There are two equivalence class …
Mikhail Borovoi's user avatar
2 votes
1 answer
86 views

Cyclic extensions of a number field of full local degree in a given set $S$

Let $K$ be a number field, and let $S=S_f\cup S_{\mathbb R}\cup S_{\mathbb C}$ be a finite set of places of $K$, where $S_f$ denotes the set of finite places in $S$, $S_{\mathbb R}$ denotes the set of …
Mikhail Borovoi's user avatar
2 votes
0 answers
97 views

Estimating the cardinality of the set of conjugacy classes of subgroups in a finite group of...

1. Let $G$ be a finite group of order $n$. I need an estimate for the number $c$ of conjugacy classes of subgroups $D\subseteq G$. Note that any subgroup of $G$ contains $1_G$, and so the set of all s …
Mikhail Borovoi's user avatar
2 votes
0 answers
62 views

Constructing a cyclic extension $L$ with given local behavior of a global field $K$ such tha...

Let $F$ be a global field without real places (that is, a function field or a totally imaginary number field). Let $K/F$ be a cyclic extension of degree $n$. Let $S$ be a ${\rm Gal}(K/F)$-invariant fi …
Mikhail Borovoi's user avatar
1 vote
0 answers
56 views

Normality in a tower of cyclic extensions of global fields, as in Artin-Tate

Let $L_0$ be a global field without real places, that is, a global function field or a totally imaginary number field, and let $V_f(L_0)$ denote the set of finite (that is, non-archimedean) places of …
Mikhail Borovoi's user avatar
4 votes
0 answers
135 views

A normal extension of a number field of given degree that does not split over a given set of...

Let $K$ be a number field and $S$ be a finite set of non-archimedean places of $K$. Let $n>1$ be a natural number. Question. Does there exist a normal extension $L/K$ of degree $n$ such that $L\otime …
Mikhail Borovoi's user avatar
1 vote

$\mathbb{Q}$-forms of $\operatorname{SL}_4(\mathbb{R})$ inside $\operatorname{SL}_8(\mathbb{...

$\newcommand{\nN}{{\mathcal N}} \newcommand{\SL}{{\rm SL}} \newcommand{\G}{{\bf G}} \newcommand{\Q}{{\Bbb Q}} \newcommand{\R}{{\Bbb R}} \newcommand{\C}{{\Bbb C}} $The answer is No. Write $\nN$ for the …
Mikhail Borovoi's user avatar
6 votes
Accepted

The second Tate-Shafarevich group of a permutation module is trivial

We write $G_w={\rm Gal}(L_w/K_v)$. Definition. For $n\ge 1$, we denote $$Ш_\omega^n(G,M)=\ker\Big(H^n(G,M)\to\prod_C H^n(C,M)\Big)$$ where $C$ runs over the cyclic subgroups of $G$. Remark. $Ш^2(L …
Mikhail Borovoi's user avatar
4 votes
2 answers
291 views

Biquadratic extension of global function fields with cyclic decomposition groups

Let $F$ be a global function field, for example $F={\mathbb F}_q(t)$, the field of rational functions in one variable over a finite field ${\mathbb F}_q\,$. Question. What would be an example of a gl …
Mikhail Borovoi's user avatar
1 vote
1 answer
180 views

Decomposition groups for the Galois module $\mu_8$

$\DeclareMathOperator{\Hom}{Hom} \DeclareMathOperator{\Aut}{Aut} \DeclareMathOperator{\Gal}{Gal} \newcommand{\Z}{{\Bbb Z}} \newcommand{\Q}{{\Bbb Q}} \newcommand{\Fbar}{{\overline F}} \newcommand{\G}{\ …
Mikhail Borovoi's user avatar

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