Questions related to the algebraic structure of algebraic integers

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Constructing Witt polynomials

I am reading about Witt vectors, and I keep seeing the following set of congruences often: For example, in these notes here, on page 3 we see the following congruences: \begin{align} ...
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2answers
66 views

Galois group is isomorphic to group of characters. What can we say then?

Let $\overline{K}/K$ denote the separable closure of a finite field $K$ of characteristic $p$ and let $\mu_{n}$ denote the group of $n$-th roots of unity in $\overline{K}$, where $(n,p)=1$. Let us ...
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An ideal of $\mathcal O_K$ [duplicate]

Let $K$ be a number field and let $\mathcal O_K$ be its ring of integers. Let $I$ be a non-zero ideal in $\mathcal O_K$. If $I$ is free as a $\mathcal O_K$-module then is it a principal ideal? ...
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If $K\cap\Bbb Q^{\text{cycl}}=\Bbb Q(\zeta_m)$ and $K/\Bbb Q$ Galois, then $\text{Gal}(K(\zeta_n)/K)\cong\text{Gal}(\Bbb Q(\zeta_n)/\Bbb Q(\zeta_m))$

$\DeclareMathOperator{\Gal}{Gal}$ Here is my argument: Induction on the number of primes dividing $n/m$. If there are two primes (i.e., $K(\zeta_n) = K(\zeta_{q_1},\zeta_{q_2})$, where ...
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Double cosets (Neukirch's Algebraic Number Theory)

This is a question from Neukirch's Algebraic Number Theory, Ch.1 $\S$9. Let $A$ be a Dedekind domain with quotient field $K$, $L$ a finite separable field extension of $K$ and $B$ the integral ...
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2answers
64 views

Show that $\mathbb{Q}(\sqrt{-14},\sqrt{-2\sqrt{2}-1})$ has degree 8 over $\mathbb{Q}$

We know that the extension $\mathbb{Q}(\sqrt{2\sqrt{2}-1}$) over $\mathbb{Q}$ has degree 4 by considering the minimal polynomial mod 3. Now I want to show that $-14$ isn't a square in this field. How ...
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3answers
97 views

Showing reducibility of a polynomial in a Discrete Valuation Ring

Let $R$ be a complete discrete valuation ring with uniformiser $\pi$. I would like to show that a polynomial $f$ in $R[X]$ is reducible. Does it suffice to show that $f$ is reducible in ...
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2answers
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ramification index in an example

Let $L=\mathbb{Q}_5[x]/(x^4+5x^2+5)$, where $\mathbb{Q}_5$ is the field of 5-adic numbers. Note that the polynomial that we are quotienting out by is an Eisenstein polynomial. So $L/\mathbb{Q}_5$ is a ...
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1answer
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Neukirch Abstract Kummer Theory. Understanding a Proof.

This question is a sort of follow up to this question, where I introduced context. Neukirch mysterious homomorphism in Abstract Kummer Theory (in his book ANT) The thing I don't understand know is ...
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$p$-adic field with infinite residue field. [duplicate]

I am reading J M Fontaine's book where on page 7 the following definition is made: A local field($K$) is a complete discrete valuation field whose reside field($k$) is a perfect field of ...
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1answer
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Algebraic number theory, Marcus, Chapter 3, Question 9

Question 9 in Marcus book. Let $K$ and $L$ be the number field such that $K\subset L$ and let $R,S$ be their algebraic integers, respectively. a) Let $I$ and $J$ be ideals in $R$, and suppose ...
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3answers
37 views

Norm of element $\alpha$ equal to absolute norm of principal ideal $(\alpha)$

Let $K$ be a number field, $A$ its ring of integers, $N_{K / \mathbf{Q}}$ the usual field norm, and $N$ the absolute norm of the ideals in $A$. In some textbooks on algebraic number theory I have ...
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1answer
52 views

Neukirch mysterious homomorphism in Abstract Kummer Theory (in his book ANT)

Someone familiar with Neukirch's terminology can understand this post better. Unfortunately it is so much terminology to just explain it here. My question is about what is marked in the picture: Why ...
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1answer
22 views

Algebraic number with conjugates having modulus 1

Suppose $\alpha$ is an algebraic number lying in a number field $K$ that is a normal extension of $\mathbb{Q}$. Suppose all the conjugates of $\alpha$ have absolute value 1. Prove or disprove that it ...
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1answer
55 views

Computing the Picard group of Number Fields in analogy with genus $0$ curves

It is possible to compute the Picard group of (some? all?) genus $0$ curves in the following manner: For concreteness let, $A = k[x,y]/(x^2+y^2-1)$ and $X = \operatorname{Spec} A$. Let $Y$ denote ...
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1answer
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Minimal polynomial of root of unity over quadratic field

Let $p$ be an odd prime and consider the $p$-th cyclotomic field $\mathbb{Q}(\zeta_p)$ and its quadratic subfield $\mathbb{Q}(\sqrt{\pm p})=:K$. I am interested in the minimal polynomial of a root of ...
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1answer
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Semilinear root of uniformiser of a p-adic field (& phi-module of Lubin–Tate formal group)

I'm looking for solutions $t$ of an equation of the form $$ t \sigma(t) \cdots \sigma^{n-1}(t) = v $$ in a field equipped with an automorphism $\sigma$ of order $n$. In this case, I call $t$ a ...
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0answers
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Enumeration of the number of splitting fields

Suppose $f(x):=x^p+ax+b\in \mathbb Z[x]$ and let $S_f$ be the minimal splitting field of $f(x)$. How can we estimate $\#\{(a,b):|S_f:\mathbb Q|=2p\}$?
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“Logarithmic derivative of a p-adic number”

Coleman, in "Division Values in Local Fields", (Inventiones math. 53, 91 - 116 (1979)), says that In his work on cyclotomic fields Kummer observed that various formal operations on power series ...
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Primes of form $a^2 + 24b^2$

For a prime number $p \neq 2$, $3$, is it necessarily the case the prime number can be written in the form $a^2 + 24b^2$ if and only if $p \equiv 1 \text{ mod }24$? I think this has to be true based ...
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1answer
58 views

Absolute Galois group of quadratric extensions

It is well known that the absolute Galois group of $\mathbb{Q}(\sqrt{p})$ and $\mathbb{Q}(\sqrt{q})$ are nonisomorphic if $p$ and $q$ are different prime numbers. See for example Szamuely's book ...
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Sign of bivariate polynomial evaluated over two algebraic numbers

I would like to compute the sign of a bivariate polynomial $f$ evaluated over two algebraic numbers $a$, $b$. The numbers are in "isolating interval representation" meaning that each one is defined by ...
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1answer
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Neukirch's Abstract CFT. Help with a proof in abstract Kummer theory.

First of all, unfortunately, writing all the notation and terminology that he uses would make this post very big. So, I'm really hoping from an answer that comes from someone that knows this book. ...
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1answer
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Algebraic Number Theory,Marcus, Chapter 2, Question 16

In question 16 of chapter 2 in Marcus Book, I have to show that $\sqrt{3}\not\in\mathbb{Q}(\alpha)$,where $\alpha=\sqrt[4]{2}$ using the trace idea. the proof starts by assuming that ...
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1answer
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$p = x^2 + xy + 3y^2$ if and only if $p \equiv 1$, $3$, $4$, $5$, $9$ mod $11$? [duplicate]

For a prime number $p \neq 11$, do we have $p = x^2 + xy + 3y^2$ for some $x$, $y \in \mathbb{Z}$ if and only if $p \equiv 1, 3, 4, 5, 9$ mod $11$? An example where this is true:$$5 = 1^2 + 1 \times ...
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Roots of unity in fixed field of decomposition group.

$\zeta_q\in L^{G({\frak P})}\Leftrightarrow$ if $\sigma\in$ $G(\frak p$) then $\sigma(\zeta_q)=\zeta_q\Leftrightarrow$ if $\sigma$ fixes $\frak P$ then $\sigma$ fixes $\zeta_q$. What's next? Suppose ...
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0answers
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Probability that the discriminant of a quadratic field is divisible by a given prime number $p.$

Find the probability that the discriminant $D$ of a quadratic field $\mathbb{Q}(\sqrt{d})$ is divisible by a given prime number $p$ (beware: the result is not what you may expect.) This is an ...
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9answers
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$p = x^2 + xy + y^2$ if and only if $p \equiv 1 \text{ mod }3$?

For a prime number $p \neq 3$, do we have that$$p = x^2 + xy + y^2$$for some $x$, $y \in \mathbb{Z}$ if and only if$$p \equiv 1 \text{ mod }3?$$I suspect this is true from looking at the example$$7 = ...
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2answers
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What does $p$-integral mean?

I'm currently studying Washington's Introduction to Cyclotomic fields and in Theorem 5.10 I came across the term $p$-integral. What does this mean? To give a bit of context: Let $n$ be even and ...
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0answers
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Siegel's article “The volume of the fundamental domain for some infinite groups”: trouble with understanding computations

This is the article I mentioned. While the idea of what Siegel is doing in order to compute the volume of the fundamental domain described in the article (the very first one, for there are discussed ...
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Abstract CFT. What does Neukirch mean with exponent here?

In Neukirch's Algebraic Number Theory, there is the following Proposition: If we take $K$ a field with characteristic $5$, $n=4$, and choose $\Delta=K^{\ast\:4}$, wouldn't it imply that the trivial ...
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1answer
32 views

Galois group of a subextension is a subgroup of the Galois group of the extension?

Let $\overline{\mathbb{F}}_{5}$ be the separable closure of $\mathbb{F}_{5}$ and let $G=G(\overline{\mathbb{F}}_{5}/\mathbb{F}_{5})$ be its Galois group. Say we pick a finite subextension of ...
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1answer
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What's the sense behind that lemma?

Please if someone can help and can take 3 minutes I would be so so unbelievably happy because it is really important to me... Thank you :) We assume we have a $m$-th root of unity ...
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1answer
33 views

Ramification of extension composition

$L, E\supset K$ are number fields. $L/K$ is normal. And field $M=LE$. Assume $\Omega$ is a prime ideal of M and its intersections with $L, E, K$ are $\mathfrak B,\mathfrak q,\mathfrak p$. ...
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1answer
54 views

Proof of Kummer's Lemma in S. Langs 'Cyclotomic fields'

I was going through the proof of Kummer's Lemma (stated below) as done in Serge Langs Cyclotomic fields on page 312. Now the author states that by class field theory it suffices to show that ...
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3answers
39 views

Extension of finite fields under norm map

If $L/K$ is a finite Galois extension with group $G$, we can define the norm of an element $a\in L$ as \begin{equation} N_{L/K}(a)=\prod_{\sigma\in G}\sigma(a). \end{equation} And obviously, this ...
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Given only the minimal polynomial of a field and approx primitive element, find subfields

I want to solve the following problem. Given: The minimal polynomial of a number field, and a decimal approximation of the field's primitive element. Compute: Is the number field an imaginary ...
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1answer
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When do imaginary quadratic extensions have unique complex places?

This question has been edited in light of some helpful comments from @AdamHughes below. Let $F$ be a totally real number field, i.e. $F=\mathbb{Q}(t)/m(t)$ where $m(t)\in\mathbb{Z}[t]$ and all roots ...
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1answer
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Decomposition and inertial fields of primes in $\mathbf{Q}(\sqrt{2},\sqrt{3},\sqrt{5})$

I recently ran into this old number theory prelim problem. Let $K=\mathbf{Q}(\sqrt{2},\sqrt{3},\sqrt{5})$ and let $\mathcal{O}_K$ be the ring of integers of $K$. Find the ramification index and ...
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Problem from the book Number Fields by Marcus

I have been stuck on the 14(c)th problem of the 3rd chapter from Marcus' Number Fields. Let $K$ and $L$ be number fields, $K \subset L$, $R = \mathbb{A}\cap K$, $S = \mathbb{A} \cap L$. Moreover ...
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Does $p = x^2 + 9y^2$ for some $x$, $y \in \mathbb{Z}$ if and only if $p \equiv 1 \text{ mod }12$?

For a prime number $p \neq 2$, $3$, does $p = x^2 + 9y^2$ for some $x$, $y \in \mathbb{Z}$ if and only if $p \equiv 1 \text{ mod }12$? A case where this is true as to suggest plausibility: $13 = 2^2 ...
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1answer
23 views

Taking $d$-th root of an element in the algebraic closure of $\mathbb{Q}$

Suppose I have $\alpha \in \overline{\mathbb{Q}}$, the algebraic closure of $\mathbb{Q}$. Suppose I was interested in an element $\beta$ such that $\beta^d = \alpha$. Does there always exist $\beta ...
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0answers
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Infinite primes (places) of a number field geometrically

Given a (global) number field $K$, thinking of the affine scheme $\mathrm{Spec}\mathcal{O}_K$ can gige an insight into (at least) some kf the number-theoretic terminology, e.g. ramification or local ...
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1answer
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Completions of number fields

I would like to prove a statement about completions of number fields, but I'm running into a problem. The statement I want to prove is Let $L/K$ be a Galois extension of number fields, $p$ a prime ...
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1answer
108 views

A prime ideal $\mathfrak{p}$ decomposes in $\mathbb{Q}(\zeta_{12})/\mathbb{Q}(i)$ iff it is generated by $\alpha\in1+3\Bbb{Z}[i]$

Prove that for a nonzero prime ideal $\mathfrak{p}$ of $\mathbb{Z}[i]$ which does not divide $3$, $\mathfrak{p}$ decomposes completely in the quadratic extension ...
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1answer
275 views

Three angles are linearly independent over $\mathbb{Q}$?

If$$\tan \alpha = 1, \text{ }\tan \beta = {3\over 2}, \text{ }\tan \gamma = 2,$$then does it follow that $\alpha$, $\beta$, $\gamma$ are linearly independent over $\mathbb{Q}$? It is possible to test ...
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4answers
109 views

Subgroup generated by $1 - \sqrt{2}$, $2 - \sqrt{3}$, $\sqrt{3} - \sqrt{2}$

For a number field $K$, Dirichlet's unit theorem says that$$(O_K)^\times = \mathbb{Z}^{r - 1} \oplus (\text{a finite cyclic group}),$$where $r$ is the number of all infinite places of $K$. An infinite ...
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1answer
17 views

irreducible in the ring of integers

There is a primitive 12-th root of unity and 5 is not a prime since the minimal polynomial mod 5 is reducible. The problem is I don't know how to show 5 is irreducible or not. What I thought was if ...
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0answers
50 views

Is $(2, \sqrt{m})$ a principal ideal or not in the ring $\mathbb{Z}[\sqrt{m}]$ [closed]

Let $m$ be a negative even integer. In the ring $\mathbb{Z}[\sqrt{m}]$, is the ideal $(2,\sqrt{m})$ a principal ideal?
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1answer
52 views

Class number of $\mathbb{Q}(\sqrt{n})$ always even? [closed]

Let $n$ be a negative square-free even integer. Does it necessarily follow that the class number of $\mathbb{Q}(\sqrt{n})$ is even?