# Questions tagged [galois-theory]

Galois theory, named after Évariste Galois, provides a connection between field theory and group theory. Using Galois theory, certain problems in field theory can be reduced to group theory, which is, in some sense, simpler and better understood.

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### Is $E$ finite and normal? [closed]

Let $E$ be a field and $H$ a finite subgroup of the group Aut $E$ of all automorphisms of $E.$ Let $F=$ Inv $H$ be the field of invariants of $H$.
Is $E$ finite and normal over $F$? Is every element ...

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### Galois groups of special polynomials

This question is motivated by long experiments with GAP.
Call a monic polynomial with integer coefficients special in case it is irreducible and has only coefficients $-1$, $0$ or $1$. Let $n \geq 5$....

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### Galois group of polynomials related to Fibonacci and Catalan numbers

Let $F_n$ be the Fibonacci and $C_n$ the Catalan numbers.
Define a polynomial by $F_n(x):=\sum\limits_{k=1}^{n}{F_k x^{n-k}}$.
For example $F_8(x)=x^7+x^6+2x^5+3x^4+5x^3+8x^2+13x+21$.
And another ...

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### When is the image of a morphism of absolute Galois groups the torsion subgroup of one of them?

Suppose $K$ and $K'$ are two characteristic zero fields of respective separable closures $K^{sep}$ and $K'^{sep}$.
Do we know all the couples $(K,K')$ such that $\mathrm{Tor}(\mathrm{Gal}(K^{sep}/K))\...

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### Number of real roots of irreducible polynomials that are solvable by radicals

Let $n \geq 3$ be a natural number. Define the set $X_n$ as the set of natural numbers that appear as the number of real roots an irreducible polynomial of degree $n$ over $\mathbb{Q}$ which is ...

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### How to prove that $\mathrm{Aut}(\mathcal{M})\cong\mathrm{Gal}(\bar{\mathbb{Q}}/\mathbb{Q})$?

I want to study the structure of the rig of L-functions $\mathcal{M}$, which is defined as the maximal set of automorphic L-functions of $\mathrm{GL}_{n}(\mathbb{A}_{\mathbb{Q}})$ for some $n$ that be ...

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### Criterion for generic polynomials

Generic polynomials, which are recalled below, play an important role in the constructive aspects of the inverse Galois problem.
Definition. Let $P(\mathbf{t},X)$ be a monic polynomial in $\mathbb{Q}...

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### Does “tensoring” with a fixed field preserve Galois extensions of finite fields?

Let $K$ be a (possibly infinite) field of characteristic $p$, and $L$ be a finite field extension of $\mathbb{F}_p$, so that $L$ is finite and $L/\mathbb{F}_p$ is Galois. Suppose $K \otimes_{\mathbb{F}...

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### “sparsifying” a binary (over the field F2) matrix

Assume I have a matrix $A \in GF(2)$, i.e., $A_{i,j} \in \{0, 1\}$ and the sum is modulo 2. Is there any known algorithms/methods to sparsify (reduce the number of non-zero entries) $A$ while keeping ...

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### Is there a Galois theory for deformations of curves?

I have some general questions about the deformations of Galois covers of curves. Suppose we are given a $G$-Galois cover $k[[z]]/k[[x]]$, where $k$ is algebraically closed of characteristic $p>0$. ...

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### Atlas of polynomial Galois groups

A polynomial $p \in \mathbb{Z}[x]$ of degree $n$ can be encoded as a finite sequence $(a_0,a_1, \dots, a_n)$, i.e. $p(x)= \sum_{i=0}^n a_i x^i$.
Let $G(a_0,a_1, \dots, a_n)$ be the Galois group of the ...

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### Given an abelian galois map of curves, what are the principal divisors on the source fixed by the galois group?

Let $f:X\rightarrow Y$ be an abelian galois map (not necessarily unramified) of nonsingular complete curves over algebraically closed $k$, where the order of the galois group $A$ is coprime to the ...

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### Properties of Mod $\ell^m$ Galois representation associated to modular form

(Sorry for my poor english..)
Let $F(z)\in S_{2k}(SL_2(\mathbb{Z})$) be a newform and $\ell$ be a prime larger than $3$. Let $K$ be a some number field and $v$ be a prime of $K$ over $\ell$. Let $K_v$...

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### How to (easily) obtain the splitting field for dihedral extensions

Let $f(x) \in {\mathbb Q}[x]$ be a polynomial that is irreducible over ${\mathbb Q}$ with $D_{n}$ (the dihedral group of order $2n$) as its Galois group. Let $\alpha$ be a root of $f(x)$ and put $K={\...

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### Cubic polynomials over finite fields whose roots are quadratic residues or non-residues

For a cubic polynomial $f(x)=x^3+x^2+\frac{1}{4}x+c$ over $\mathbb{F}_q$, where $q$ is a odd prime power, I find that for a lot of $q$, there does not exist $c\in\mathbb{F}_q$ such that $f$ has three ...

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### Any way around Abel's impossibility theorem?

Abel's impossibility theorem states that the roots of a general polynomial (of degree 5 or higher) cannot be written using arithmetic operations and radicals. Radicals are solutions of a specific ...

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### Conjugation action of $Gal(\bar{s}/s)$ on the tame ramification group

There is a statement in SGA 7-1 Exposé 1 (P. Deligne, Résumé des premiers exposés de A. Grothendieck, pdf of SGA7-1), (0.3.1):
$S$ is a Henselian trait (i.e. the spectrum of a henselian discrete ...

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### Algorithm to compute minimal polynomials

Suppose $L/K$ is a finite Galois extension of fields of degree $n$. Suppose that we know an irreducible polynomial $f\in K[x]$ such that $L\cong K[x]/(f)$. Suppose also that we know the Galois group ...

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### Non-linear Galois descent

This question is about Galois theory. So let $K / k$ be a Galois extension of fields. Let us assume that $K / k$ is finite dimensional, though everything can be made to work in the profinite case by ...

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### Lang's proof of the Shimura's exact sequence

Let $E$ be an elliptic curve having invariant $j$ and defined over $\mathbf C(j)$. Let $\sigma$ be an automorphism of $F_{\mathbf C}$, the field of all modular functions of all levels. Let $p$ be a ...

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### Explicit construction of abelian wild inertial extensions of maximal tamely ramified extension of $\mathbb{Q}_p$?

In Iwasawa's paper On Galois groups of local fields, he proves that if $V$ is the maximal tamely ramified extension of $\mathbb{Q}_p$, with Galois group $\Gamma$ over the base, then its abelianized ...

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### split integral model of a reductive group

Let $F$ be a number field, $p\in\mathbb{Z}$ a prime which is unramified in $F$ and $G$ a connected reductive group over $F$. Moreover $G$ is supposed to be quasi-split over $p$.
Does there exist a ...

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### Splitting field of an intermediate field

Consider the following 'wrong' question.
Let $f(x) \in F[x]$ be an irreducible polynomial in a polynomial ring of a field $F$. Let $L$ be the splitting field of $f(x)$ over $F$. Assume that $L$ is a ...

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### Fontaine - Wintenberger field of norms and imperfect case

Let $K$ be a complete discrete valued field whose residue field $k_K$ has characteristic $p$ and has the property that $[k_K:k_K^p]=p^d$ for some $d$. Let $t_{\alpha}, 1 \leq \alpha \leq d$ be a set ...

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### Local Class field theory for curves over $p$-adic fields

Let $X$ be a smooth curve over $\operatorname{Spec}\mathbb{Q}_p$ and $P\in X(\mathbb{Q}_p)$. Let $K_P\simeq \mathbb{Q}_p((T))$ denote the completion of the function field of $X$ at $P$. What is known ...

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### p-adic field extension of degree 2n without a subfield of degree 2?

I need an example of a p-adic field extention $L/F$ of degree $[L:F]=2n$ without a subfield $K\subset L$ of degree $[K:F] = 2$.

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### Collinear Galois conjugates

This is inspired by this old question, which may provide a bit more background. But the two present questions seem somewhat more fundamental to me.
Let $p$ be an irreducible polynomial with integer ...

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### Non-cyclic Galois groups over the field of formal Laurent series in positive characteristic

This should be an easy question, but I am unfortunately not able to give an answer, so I am sorry if this is not the appropriate level for the site.
Let $C$ be an algebraically closed field of ...

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### Abel-Ruffini theorem for systems of polynomial equations

I know the Abel-Ruffini theorem, which states that a general polynomial equation in one variable with degree $\geq 5$ is not solvable in radicals. I wonder whether there is a similar result for ...

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### Linear independence of approximate square roots

From Galois theory, we know that $[\mathbb{Q}(\sqrt{2}, \sqrt{3}, \dots \sqrt{p_k}) : \mathbb{Q}] = 2^k$. Suppose I plug in rational approximations to the square roots, then of course the classical ...

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### Is there an algorithm to compute a Belyi map for the Riemann surface?

Let $y^2=x^5-x-1$ be an affine model of a projective complex curve, is there an algorithm to compute the Belyi map (preferably of small degree), i.e., map to the projective line ramified only at $\{0,...

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### Galois theory and Lie theory [closed]

Galois theory talks about solving algebraic equations by radicals and the motivation for Lie to develop his theory is to study differential equations.
I want to understand what is happening in common ...

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**1**answer

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### Enumeration of field generators of a finite field over $\mathbb{F}_{q}$, which are $m^{th}$ powers in the same field

Consider the finite field extension $\mathbb{F}_{{q}^{d}}$ over $\mathbb{F}_{q}$, where $q=p^a$ for some prime $p$. We assume $d\geq 2$. Let,
$$ S=\{ \alpha \in \mathbb{F}_{q^d}\hspace{0.1 cm} | \...

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### Intrinsicness of Hodge-theoretic properties of Galois representations in a general reductive group

In the paper "The conjectural connections between automorphic representations and Galois representations" by Buzzard and Gee, it is said
"We say that
$\rho$ is crystalline/de Rham/Hodge–Tate if ...

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### Name and properties of this combination of group algebra and semidirect product?

Given a field $k$, a group $G$, and a homomorphism $\phi : G \to \mathrm {Aut} (k)$, we can define a ring $\widehat {k [G]}_\phi$ as follows: As an abelian group it is isomorphic to the group algebra $...

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### Exact differential forms in characteristic $p>0$

Let $k$ be an algebraically closed field of characteristic $p>0$. Suppose $1< e_i <p$ for $i=1,2, \ldots, n$ are integers ($n \ge 2$). What are the conditions on the $e_i$'s so that the ...

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### Galois action on Grothendieck ring of varieties

Let $k$ be a field and let $\overline{k}$ be its algebraic closure. Let $K_0(Var/\overline{k})$ be the Grothendieck ring of algebraic varieties over $\overline{k}$.
Is it true that the natural ...

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### Galois elements determined by action on $n$-th roots of rationals?

Can there be an element $\sigma$ of $Gal(\overline{\mathbb{Q}}/\mathbb{Q})$, other than the identity and complex conjugation, which is completely determined up to conjugacy by its action on $\sqrt[n]r$...

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### Weyl theorem for non specified primitive root of unity

Let $\omega=e^{2i \pi/p}$.
Weyl theorems give all representations of matrix algebra span by $A,B$ such that either
$AB=\omega BA, A^p=B^p=I$,
or
$(k,l)\mapsto A^kB^l$ is a irreducible ...

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### Monodromy groups from Galois's viewpoint

According to the Wikipedia article about monodromy, the monodromy group can be defined in terms of Galois theory in following way:
Let $F(x)$ denote the field of the rational functions in the ...

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### Earliest use of the term “Galois extension”?

Does anyone know the earliest use of the term "Galois extension"? I thought it might be in Emil Artin's Notre Dame lectures but I couldn't find it there. (He does use the terms "normal" and "separable....

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### Nature of polynomials of the form $x^n-a$ over finite fields

I state the following theorem from Serge Lang's Book- Algebra(3rd edition).
Theorem: Let $k$ be a field and $n$ an integer $\geq$ 2. Let $a\in k, a\neq 0$. Assume that for all primes $p$ such that $...

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### Dimension of fixed vectors of a semi-linear operator

Let $L$ be a field with a field embedding $\sigma:L \rightarrow L$, and $K=L^{\sigma}$ be the fixed field of $\sigma$. For $A \in M_n(L)$ a matrix, consider the set $X=\{x \in L^n|Ax=\sigma(x) \}$ ...

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### Dedekind's original proof of independence theorem

The independence theorem says, roughly, that a family of $\sigma\in\mathrm{Aut}_k(K)$ is $K$-linearly independent. In his Algebra, Serge Lang gave a proof of independence theorem following Artin (...

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### Classification of mod p Galois Representations for l not equal to p

Let $l\neq p$ be primes and let $\text{G}_l:=\text{G}_{\mathbb{Q}_l}$. Let $k$ be a finite field of characteristic $p$ and $\bar{\rho}:\text{G}_l\rightarrow \text{GL}_2(k)$ a local Galois ...

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### Analogy between metric space completion and algebraic closure

I've noticed some similarities between the story of completing a metric space and taking algebraic closure of a field. My question is whether these two stories can be generalized.
Metric space
Fix a ...

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### Number Rings and (Galois) Descent

In algebraic number theory, one chooses for each finite étale $\mathbb{Q}$-algebra $K$ a finite $\mathbb{Z}$-algebra $\mathcal{O}_K$. Usually one simply speaks of the finite $\mathbb{Q}$-algebras ...

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269 views

### Connection between Fourier analysis and Galois theory

Let $x\ \%\ m$ be the residue of $x$ modulo $m$, i.e.
$$x \equiv x\ \%\ m\pmod{m}$$
Let $\mu^n_m(x)$ denote multiplication by $n$ modulo $m$, i.e.
$$\mu^n_m(x) = nx\ \%\ m$$
Consider the Fourier ...

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### Galois descent for profinite groups acting on local fields

Suppose that $G$ is a profinite group acting faithfully on a field $L$ and assume moreover that the action is admissible, that is, that every $l \in L$ is stabilized by an open subgroup of $G$. If ...

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363 views

### Questions concerning the Fourier analysis of $ nx\ \%\ m$

Let $x\ \%\ m$ be the residue of $x$ modulo $m$, i.e.
$$x \equiv x\ \%\ m\pmod{m}$$
The plots of the functions $f_{nm}(x) = nx\ \%\ m$ exhibit characteristic patterns, especially periods of length $...