Questions tagged [abstract-algebra]

For questions about monoids, groups, rings, modules, fields, vector spaces, algebras over fields, various types of lattices, and other such algebraic objects. Associate with related tags like [group-theory], [ring-theory], [modules], etc. as necessary to clarify which topic of abstract algebra is most related to your question and help other users when searching.

Abstract algebra is the study of algebraic objects, i.e. sets endowed with one or more operations on the elements of those sets. In particular, the study of abstract algebra considers the algebraic structures and properties of which such operations induce. It can be considered as the generalization of the study of the algebraic structure of the integers and real numbers (arithmetic), or the study of matrices and vector spaces (linear algebra).

Some algebraic objects are monoids, groups, rings, fields, vector spaces, modules, algebras, and categories, among many other less prominent objects.

Examples

  1. The set of non-negative integers $\mathbb{N} = \{0,1,2,3,\dotsc\}$ is a monoid under the operation $+$.

  2. The integers $\mathbb{Z} = \{\dotsc,-1,0,1,\dotsc\}$ under the binary operation of $+$ form a group.

  3. Furthermore, $\mathbb{Z}$ has the structure of a ring when you consider it as being equipped with both addition and multiplication.

  4. The real numbers $\mathbb{R}$ with their usual addition and multiplication form a field.

  5. The set of $n\times n$ matrices with entries in $\mathbb{R}$ with matrix addition and multiplication form a ring.

  6. The set of $1\times n$ vectors over the real numbers, with vector addition, and multiplication by elements of the $n\times n$ real matrices on the right are an example of a module for the ring of matrices.

In addition to studying the objects themselves, abstract algebra considers homomorphisms between the objects and various constructions and tools, which are useful for studying the objects.

85022 questions
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1 not equal to 0 stated as a condition, why?

In Lindsay Childs' Algebra text (3ed pg 141), it statements this proposition regarding a ring homomorphism: $ Let f: R \rightarrow S$ be a homomorphism where R is a field and $1 \ne 0$ in $S$. Then $f$ is one-to-one. My confusion is about $1 \ne 0$.…
user8969
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number of non-abelian groups of finite order

Can you say how to find number of non-abelian groups of order n? Suppose n is 24 ,then from structure theorem of finite abelian group we know that there are 3 abelian groups.But what can you say about the number of non-abelian groups of order…
Sankha
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A negation of definition

Let $R$ a ring. We know that an ideal $I$ of $R$ is said prime if for all $a,b\in R$ $$ab\in I\Rightarrow a\in I\quad\text{or}\quad b\in I.$$ When an ideal is not prime? That is, what is the negation of this definition formally? EDIT I understood…
Jack J.
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Is there a finite-dimensional algebra having a non-trivial ideal in each hyperplane?

Let $A$ be a finite-dimensional associative commutative algebra with unity over a field $k$. Is it possible that every hyperplane of $A$ (where we consider $A$ as a vector space over $k$) contains a non-trivial ideal? When $A$ is semisimple (i.e.…
Dmitry
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Why does semidirect product give a complete classification of small groups

The key theorem involved is: Suppose $G$ is a group with subgroups $H$ and $k$ such that $H$ is normal in $G$ and $H \cap K=1$. Let $\phi: K \to Aut(H)$ be the homomorphism defined by mapping $k \in K$, TO THE automorphism by conjugation by $k$.…
Alex
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Proving $H$ is a subgroup of $GL(2,\Bbb{R})$

Let $G=GL(2,\mathbb{R})$ and $$H=\Bigg\{{\begin{bmatrix} \cos{\theta} & \sin{\theta}\\ -\sin{\theta} & \cos{\theta} \end{bmatrix}} : \theta \in \mathbb{R}\Bigg\}$$ Prove that $H$ is a subgroup of $G$. So far I have that the identity element…
Reety
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length of modules in arbitrary exact sequences

Let $R$ be a commutative, noetherian ring. Given the exact sequence of $R$-modules of finite length $ 0 \rightarrow M_0 \rightarrow M_1 \rightarrow M_2 \rightarrow M_3 \rightarrow 0 $. Is there an equation, connecting the lengths of the modules…
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Cantor's theorem with proof and example

I've just saw the Cantor's theorem some days ago, but I really can't get my head around the proof. I read everywhere the same thing on Wikipedia, YouTube, and in class. The only thing I know that it is to be proved by contradiction and that we are…
Valentin
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How do I define Polynomial ring $R[x]$ over a commutative ring $R$ with unity?

I have struggled to define "Polynomial Ring" today. Since I'm not familiar with abstract algebra, i don't know if there is a theorem states that "For every commutative ring $R$ with unity, there exists a topology on $R$". I'm wondering this, because…
Katlus
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Surjective homomorphisms $\mathbb{Z} \oplus \mathbb{Z} \rightarrow \mathbb{Z} \oplus \mathbb{Z}$ are isomorphisms.

Is there an easy way to prove that any surjective homomorphism $\mathbb{Z} \oplus \mathbb{Z} \rightarrow \mathbb{Z} \oplus \mathbb{Z}$ is an isomorphism? I was told that this holds more generally for any direct sum of $\mathbb{Z}$. This is easy in…
Tuo
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Degree of $F(x)$ over $F(y)$ where $y=f(x)/g(x)$ is a rational function.

A problem states: Let $x$ be a transcendental element over field $F$, and $y = f(x)/g(x)$ be a rational function, with relatively prime polynomials $f,g \in F[t]$. Let $n = \max(\deg f,\deg g)$. Suppose $n \ge 1$. Prove that $[F(x) : F(y)] = n$. By…
Y.Guo
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Construction of $F(a)$ as a field of fractions.

$F$ is a field. $a$ is in some extention $E$ of $F$ and $a$ is transcendental over $F$. I think that $F(a)$, the smallest subfield of E that contains both $F$ and $a$ can be constructed in such a way. Let $A=\{f(a)|f(x) \in F[x]\}$. If we view E as…
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Ternary quartics with trivial automorphism group

Let $f\in\mathbb{C}[x,y,z]_4$ be a smooth ternary quartic. And let $G$ be the automorphism group of $f$ (those elements in $GL_3(\mathbb{C})$ fixing the curve $f=0$). For example in Dolgachev's book (http://www.math.lsa.umich.edu/~idolga/CAG.pdf)…
blacky
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If $f:R \rightarrow R$ is an epimorphism, is $\hat f:\mathcal{M}_2 (R) \rightarrow \mathcal{M}_2 (R)$ an epimorphism?

Suppose $f:R \rightarrow R$ is an homomorphism between rings. Consider the function $\hat f:\mathcal{M}_2 (R) \rightarrow \mathcal{M}_2 (R)$ such that $\hat{f}(a_{ij})=(f(a_{ij}))$. Show $\hat{f}$ is an homomorphism. If $f$ is an epimorphism, does…
Yagger
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Division by zero restores values

First of all, I am aware of all the laws of algebra , so my question is about intuition . We know that $\frac{x}{y}$ undoes the operation $x \times y$ . Shoudn't $(x/0)$ undo $x \times 0$ ? That means , if $x \times 0$ destroys x , maybe…
Youssef
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