Questions tagged [field-theory]

Use this tag for questions about fields and field theory in abstract algebra. A field is, roughly speaking, an algebraic structure in which addition, subtraction, multiplication, and division of elements are well-defined.

This tag is NOT APPROPRIATE for questions about the fields you encounter in multivariable calculus or physics. Use (vector-fields) for questions on that theme instead.

In mathematics, a field is a set on which addition, subtraction, multiplication, and division are defined, and behave as when they are applied to rational and real numbers. A field is thus a fundamental algebraic structure.

An archaic name for a field is rational domain. The French term for a field is corps and the German word is Körper, both meaning "body." A field with a finite number of members is known as a finite field or Galois field.

Because the identity condition is generally required to be different for addition and multiplication, every field must have at least two elements. Examples include the complex numbers ($\mathbb{C}$), rational numbers ($\mathbb{Q}$), and real numbers ($\mathbb{R}$), but not the integers ($\mathbb{Z}$), which form only a ring.

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When are nonintersecting finite degree field extensions linearly disjoint?

Let $F$ be a field, and let $K,L$ be finite degree field extensions of $F$ inside a common algebraic closure. Consider the following two properties: (i) $K$ and $L$ are linearly disjoint over $F$: the natural map $K \otimes_F L \hookrightarrow KL$…
Pete L. Clark
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Proof that an integral domain that is a finite-dimensional $F$-vector space is in fact a field

I'm reading Galois Theory by Steven H. Weintraub (second edition), and finding that I'm at least somewhat short on the prerequisites. However the following proof looks wrong to me - am I misunderstanding something, or is it actually an incorrect…
35
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Is it true in an arbitrary field that $-1$ is a sum of two squares iff it is a sum of three squares?

Here's a statement from Lam's First Course in Noncommutative Rings. (Paraphrased) Let $k$ be a field. Then the following conditions are equivalent. $$(\forall a,b,c,d\in k)\;\;(a,b,c,d)\neq 0\implies a^2+b^2+c^2+d^2\neq 0;\tag 1$$ $$-1\text{ is not…
user23211
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A proof of Artin's linear independence of characters

I came up with a proof of Artin's linear independence of characters in field theory. The usual proof uses a clever trick devised by Artin. Since I'm not as clever as him, I prefer a proof which doesn't use a clever trick. Is this proof well-known?…
Makoto Kato
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Does there exist a field $(F,+,*)$ so that $(F,+) \cong (F^*,*)$?

This question occurred to me earlier today. I can see that if the field has a unit, then there is an element of multiplicative order $2$, namely $-1$. Thus if there was an isomorphism $(F,+) \cong (F^*,*)$ then the characteristic of the field would…
Elle Najt
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18
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Quadratic extensions in characteristic $2$

I recently saw in class that the degree $2$ extensions of a field of characteristic $\neq 2$ are given by square roots of non-squares in the base field. I wonder what happens in the case of characteristic $2$ fields. For finite fields of…
JessicaB
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Can a field be isomorphic to its subfield?

Let $K$ be a field and $K(X)$ be the field of its rational functions. Now let $\phi \in K(X)$ be a rational function such that $K(\phi) \neq K(X)$. Now, since $\phi$ is transcendental over $K$, $K(\phi)$ is isomorphic to $K(X)$. Is this a correct…
Mohan
  • 14,856
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3 answers

Is the Pythagorean closure of $\mathbb Q$ equal to the field of constructible numbers?

A Pythagorean field is one in which every sum of two squares is again a square. $\mathbb Q$ is not Pythagorean, which is easy to see. I have read a theorem online which says that every field has a unique (up to isomorphism) Pythagorean closure. I…
user23211
15
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Does every algebraically closed field contain the field of complex numbers?

Does every algebraically closed field contain the field of complex numbers? Thank you very much.
LJR
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Are the real numbers a nontrivial simple extension of another field?

Is there a proper subfield $K$ of the real numbers and a real number $\theta$ such that $\mathbb R = K(\theta)$? I thought of this question earlier idly wondering about what the structure of the poset of all subfields of $\mathbb C$ looks like and…
Mees de Vries
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What is $\mathbb{C}^{\mathrm{Aut}(\mathbb{C}/\mathbb{Q})}$?

Let $\mathrm{Aut}(\mathbb{C}/\mathbb{Q})$ be the field automorphisms of $\mathbb{C}$, and $\mathbb{C}^{\mathrm{Aut}(\mathbb{C}/\mathbb{Q})}$ the subfield of $\mathbb{C}$ fixed by this group. I supsect that it is equal to $\mathbb{Q}$ but I have…
user10676
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12
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Faulty definition of a field in Curtis' Abstract Linear Algebra?

On pp.2-3 of Curtis, Abstract Linear Algebra, he gives a definition of a field which seems to fail to exclude a pathological example. He says a field is a set k with two operations (a+b) and (ab) such that: $k$ is an abelian group with + $k - {0}$…
nham
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Is $\sqrt{2}\in\mathbb{Q}(\sqrt[8]{3})$ or not?

My hunch is that $\sqrt{2}\not\in\mathbb{Q}(\sqrt[8]{3})$. For practice, I want to compute the splitting field and its degree of $x^8-3$ over $\mathbb{Q}$. I know the roots are $\sqrt[8]{3},\sqrt[8]{3}\omega,\dots,\sqrt[8]{3}\omega^7$ where…
yunone
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What kinds of non-zero characteristic fields exist?

There are these finite fields of characteristic $p$ , namely $\mathbb{F}_{p^n}$ for any $n>1$ and there is the algebraic closure $\bar{\mathbb{F}_p}$. The only other fields of non-zero characteristic I can think of are transcendental extensions…
Dinesh
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10
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Can I embed $\Bbb{C}(x)$ into $\Bbb{C}$?

Is it possible to embed $\Bbb{C}(x)$ (the field of rational functions over the complex numbers) in $\Bbb{C}$ ? Thank you!
Nemes
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