Galois theory allows one to reduce certain problems in field theory, especially those related to field extensions, to problems in group theory. For questions about field theory and not Galois theory, use the (field-theory) tag instead. For questions about abstractions of Galois theory, use ...

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Factorisation of large polynomials and Galois theory

As I understand it, one of the consequences of Galois theory is that there is no way of expressing the solutions to a general polynomial of degree 5 or higher in terms of radicals. Would a theory that ...
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On the expression of the Galois conjugates in terms of the coordinates in a basis

Let $K$ be a field and let $L$ be a Galois extension of $K$. Assume that $[L:K]=n$, and consider $e=(e_1, e_2, ...,e_n)$ a basis of $L$ over $K$. We note ...
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How to find minimal polynomial of primitive element (field theory)

I am given a primitive element $\alpha$ in the Galoisfield $F_{2^6}$. The question is to find the mimimal polynomial of $\alpha^7$. How to I find this? My thoughts so far: $$ \alpha^7 \rightarrow ...
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Irreducible Polynomials, and Galois Groups

I have a small question about some concepts. I know that if I have a polynomial $p(x)$ over a field $K$ and an extension field $F$, that elements of $\text{Gal}(F/K)$ permute the roots of $p(x)$ ...
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Why are inertia and decomposition groups only defined over normal extensions?

Can't we define them as the subgroups of the automorphism group of an arbitrary extension $L/K$ that fixes a prime $Q$ in $\mathcal O_L$ over a prime $P$ in $\mathcal O_K$?
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What is the gcd of $x^3-2$ and $x^2+3ax+3a^2$?

I'm trying to understand Galois theory. $f(x)= x^3-2$, with $a$, $b$ and $c$ as roots. Galois resolvent $V_0= a+2b+3c$, $V_1= c+2a+3b$,... etc till $V_5=a+2c+3b$ assume $X=V_0$ ...
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Splitting field and galois group of $x^4-2x^2-1$ over $\Bbb{Q}$

I have that the roots of $m(x)=x^4-2x^2-1$ are plus minus $a=\sqrt{1+\sqrt{2}}$ and $b=\sqrt{1-\sqrt{2}}$ which is complex because $1-\sqrt{2}$ is negative. Taking out $i$ and letting $b=ic$ we have ...
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Extension of fields $[L:K]=2$. Show that there is a element $\alpha\in L$ such that $L=K(\alpha)$ and $\alpha^2\in K$.

Let $K$ be a field with characteristic different from two, then if a extension of fields $L/K$ is such that $[L:K]=2$ then there is a element $\alpha\in L$ such that $L=K(\alpha)$ and $\alpha^2\in K$. ...
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Isomorphism type of the Galois group

$f=(x^2-2)(x^3-3)$. Let $K$ be the splitting field of $f$ over $\mathbb{Q}$. a) Determine the degree of extension of $K$ over $\mathbb{Q}$. b) Determine the isomorphism type of the Galois group of $K$ ...
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Showing polynomial is irreducible over field containing roots of unity.

Given a field $F$ containing all the roots of unity I'm trying to show that $f(x) = x^p - \alpha^p$ is irreducible over $F$ (where $\alpha$ is not in $F$). It's clear that $f$ splits in $F(\alpha)$ ...
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An introduction to monodromy in topology and algebra

I'm looking for some basic references on monodromy and monodromy groups, in particular I'd like something which describes the interplay between the topological definition (in the theory of covering ...
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Determine the galois group of a quartic

I'm reading Hungerford's algebra chapter about galois theory. There is the following theorem in p.273 (with some minor changes) about determining the Galois group of a quartic: Let $K$ be a field ...
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Galois group of $(T^4-3)(T^6-3)$

Given the polynomial $f(T) = (T^4-3)(T^6-3)$, I would like to calculate the Galois group of $f$. What I've done is the following: setting $\alpha = 3^{1/4}$ and $\beta= 3^{1/6}$, $\zeta_k = e^{2\pi i ...
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Relations between galois group of polynomial and its factors

$f = f_1\dots f_n$ where $f_i$ is irreducible and distinct. What can i say about $\operatorname{Gal}(f/\mathbb{Q})$ if i know $\operatorname{Gal}(f_i/\mathbb{Q})$ for any(some) $i$.
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Galois group of $x^6-5x^3+6$

Let $f = x^6-5x^3+6$. I want to determine $\operatorname{Gal}(f/\mathbb{Q})$ without some group theory tricks (like Sylow's theorems) and without reduction $\bmod p$. Let $L_f = ...
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Galois group of $x^6-9$

$f = x^6-9 = (x^3-3)(x^3+3)$ Let $L_f$ be splitting field therefore $L_f = \mathbb{Q}[\sqrt[3]{3},e^{\frac{2\pi i}{3}}]$, $[L_f:\mathbb{Q}] = 9$. Also $Gal\space x^3±3/\mathbb{Q} = S_3$ and $Gal\space ...
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Prove the form of a fixed field

let $C_3=\langle\sigma\rangle$ and let $\sigma$ act on $K(s,t)$ by $s \mapsto t$ and $t \mapsto -s-t$ I want to prove that $K(s,t)^{C_3}=K(u,v)$ with $u=\frac{s^2+t^2+st}{st(s+t)}$ and ...
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Compositum of normal extensions is a normal extension

I'm trying to prove that if $ F \subset K, F \subset M $ are normal extensions, $ K,M \subset E $, then $ KM$ is also a normal extension of $ F $. I tried using the fact that $ F \subset KM $ is a ...
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Determine the Galois group of $ F(x^5) \subset F(x) $

I'm rather new to Galois theory and have been given this exercise: Suppose $ F $ is respectively equal to $ \mathbb{Q}, \mathbb{C}, \mathbb{F}_5 $ (the third one is just the 5-element field). My task ...
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Link between Riemann surfaces and Galois theory

In my notes for a Geometry of Surfaces course that I'm studying, there is the following quote: (For those of you who like algebra and Galois theory) Studying compact connected Riemann surfaces is ...
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Summary of Galois quintic unsolvability

Is this a sufficient flow of logic? Summary: Consider a fifth degree polynomial that has a formulaic solution. Then we have a radical extension with a string of subgroups of the Galois group which ...
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Show that $\mathbb{Q}(\sqrt p \mid p \text{ is prime})$ is an algebraic and infinite extension of $\mathbb{Q}$

Show that $\mathbb{Q}(\sqrt p \mid p \text{ is prime})$ is a algebraic and infinite extension on $\mathbb{Q}$. Well, if i consider for every $p$ prime, the polynomial $p(x)= x^2 - p$, then $p(x)$ is ...
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Galois of $x^3+2$

the splitting field of $x^3+2$ is $E=\mathbb Q(\sqrt[3]{2},i\sqrt{3}$) and I know the $Gal(E:\mathbb Q)$ has six elements which is isomorphic to $S_3$. How to find all intermediate subfields using ...
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Reference request: Cohomology of Elliptic Curves

Is it true that the group $$H^1(Gal(K^{ab}/K)/\mu_{\nu}(Gal(K_{\nu}^{ab}/K_{\nu})),E_{p^n})$$ is always p-divisible? Or are there any conditions which, when satisfied, guarantee its p-divisibility? ...
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Galois theory question (Kummer's Theory)

This question is from a past exam paper. Suppose $E = \mathbb{Q}(\theta)$, where $\theta$ is a root of $X^{3} - 39X + 26 \in \mathbb{Q}[X]$. Prove that $E$ is cyclic Galois extension of ...
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Classification of all subrings

Let $R$ be an integral domain whose underlying additive group is finitely generated free and whose field of fractions $K$ is a finite Galois extension of $\mathbb{Q}$. Is there a method of ...
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If a field extension contains a cyclotomic extension is it solvable?

This doesn't seem like it should be true but I'm not entirely sure - I would appreciate anyone looking over the following: If we have a finite Galois field extension $L/K$ and $M \subset L$ a ...
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Prove the Galois Group is Isomorphic to $S_3$

Prove G=$Gal(\mathbb{Q}(\sqrt[3]{2},i\sqrt{3}) : \mathbb{Q})$ is isomorphic to $S_3$ I know that the G has 6 automorphims, and $S_3$ has order 3! then consider polynomial $x^3-2 = ...
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$\operatorname{Gal}(\overline{K}/K)=\mathbb{Z}$ possible?

Is it possible to have $\operatorname{Gal}(\overline{K}/K)=\mathbb{Z}$? My question comes from the link beetween covering and field extensions. For covering the simplest example is ...
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Transitivity of the discriminant of number fields

Let $M/L/K$ be a tower of number fields with discriminant of $M/K: d_M$ and of $L/K: d_L$. I would like to find a transitivity theorem for the discriminant and by letting $p_i$ and $q_i$ be integral ...
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Ramification in $\mathbb Q(\zeta_5, \sqrt[5]2)/\mathbb Q(\zeta_5)$

Let $F=\mathbb Q(\zeta_5,\sqrt[5]2)$ and $K=\mathbb Q$ where $\zeta_5$ is a primitive $5$th root of unity and let $p=73$ be a prime in $K$. Fix primes $\mathfrak p$ and $\mathfrak q$ above $73$ in ...
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What is the automorphism group of the field $\mathbb{Z} /p\mathbb{Z}(t)$?

Here, $t$ is transcendental over $\mathbb{Z} /p\mathbb{Z}$. How big is this group? What are its elements? Is for example the map $t \to -t$ an automorphism?
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Automorphisms (in the context of Galois Theory)

Can someone give an explanation of what an automorphism is, in the context of Galois Theory? I keep thinking it is the set of maps which send roots of polynomials to their conjugates but I feel that ...
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Does all splitting fields have characteristic 0?

Does all splitting field have characteristic 0? This may be a bad question, but I am wondering because the author of a book I am reading summarises a lot of properties when he is about to start with ...
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Showing an extension is not normal. [closed]

Show that the extension $ \mathbb{Q}(\sqrt[3]{2}) \supset \mathbb{Q} $ is not normal.
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Galois Group, Field Extension Prove Abelian

Let F be an extension of $\mathbb{Q}$ and let $\omega = \cos{(\frac{2\pi}{n})}\sin{(\frac{2\pi}{n})}$. Prove that $Gal(F(\omega):F)$ is abelian. I am looking for a sketch of this proof. so far in ...
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Symmetric polynomials and g non symmetric

If $g=x_1+2x_2+3x_3, s_1=x_1+x_2+x_2, s_2=x_1x_2+x_1x_3+x_2x_3$ and $s_3=x_1x_2x_3$ , write $x_1, x_2$ and $x_3$ in function of $g, s_1, s_2$ and $s_3$.
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Dimension of compositum of two fields, one of them Galois.

Let $L, K, F = L \cap K$ be fields such that $[L:F] , [K:F] < \infty$. $LK$ is defined as the compositum of the two fields(shortest field containing $K$ and $L$ in some algebraic closure). Also, ...
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Cohomologies of Galois group of field extension

Let $k\subset K$ be a finite Galois extension with Galois group $G=\text{Aut}_k\,K$. How to prove that $H_i(G,K)=H^i(G,K)=0$ for all $i>0$?
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Degree of $\mathbb{Q}(\omega)/\mathbb{Q}$ where $\omega^{3}=1$

I am working through Rotman's Galois Theory, and I came across an example that confused me a bit. Here is a screenshot of the problem: I am not sure, why the degree of ...
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Primitive element theorem, simple extension

Let $X$, $Y$ be indeterminates over $F_2$, the finite field with 2 elements. Let $L = F_2(X, Y )$ and $K = F_2(u, v)$, where $u = X + X^2$, $v = Y + Y^2$. Explain why $L$ is a simple ...
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$A_n$ as a Galois group

I am looking for a clear, detailed proof that the alternating group $A_n$ is realizable as a Galois group over the rationals $Q$. (I have seen proofs using Hilbert's irreducibility theorem, but they ...
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Is the compositum of $L_1$ and $L_2$ equal to $L_1[L_2]$?

In a course about Galois theory, there is the following definition : Let $L_1$ and $L_2$ be two subfields of the field $L$. We define the compositum $L_1L_2$ of $L_1$ and $L_2$ as the smallest ...
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Galois extension over power series fields

Let $K$ be a field, and $L$ be an algebraic extension of $K$. I think it is known that if $T$ is a finite extension of $K((X))$, then $T$ is complete with respect to the $X$-adic valuation, hence if ...
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Showing that $f(x) = \frac{1}{2} x^3 + 3x^2 -\frac{5}{3} x - 5$ irreducible over $\mathbb{Q}(\sqrt[4]{5})$

I am unsure how to show that the polynomial $f(x) = \frac{1}{2} x^3 + 3x^2 -\frac{5}{3} x - 5$ is irreducible over $\mathbb{Q}(\sqrt[4]{5})$. If it were reducible, it would have a root in ...
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Does an inseparable extension have a purely inseparable element?

Assume $K/F$ is an inseparable extension. Is it necessary that K contains an element $u \notin F$ that is purely inseparable over $F$? I also posted in MO.
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Whether an extension is separable or not

It is a well known conclusion that if $K/F$ is a finite separable extension, then $K^{Emb(K/F)}=F$ (By which I mean the subfield of K fixed by $Emb(K/F)$). I am wondering, whether the inverse is true. ...
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How is this $K$-automorphism well-defined?

I'm currently reading Hungerford algebra's chapter about Galois theory, I cannot understand the following example Let $F=K(x)$ with $K$ any field. For each $a\in K$ with $a\neq 0$ the map ...
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The splitting field of $x^7 - 2$ and its Galois group

This question is more or less a "is it me or is it something wrong in the answer sheet"-question. In one of the previous exam sheets at my university, I am supposed to find the order of the Galois ...
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1answer
66 views

Calculate the degree of the extension $[\mathbb{Q}(\cos(\frac{2\pi}{p})):\mathbb{Q}]$

Calculate the degree of the extension $[\mathbb{Q}(\cos(\frac{2\pi}{p})):\mathbb{Q}]$ where $p$ is a prime number. My thoughts are: I am lost My intuition says it has to be $ \frac{p-1}{2}$ and ...