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.

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Difference between supremum and upper bounds and between infimum and lower bounds

I'm having some difficulties catching the difference between upper bound and supremum and, similarly, between lower bound and infimum. Let's take a look at this set: $A=\{x\in \mathbb Q | 0
haunted85
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Subgroups of finite solvable groups. Solvable?

I am attempting to prove that, given a non-trivial normal subgroup $N$ of a finite group $G$, we have that $G$ is solvable iff both $N$, $G/N$ are solvable. I was able to show that if $N,G/N$ are solvable, then $G$ is; also, that if $G$ is solvable,…
Cameron Buie
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Problem about the definition of Euclidean domain

In the definition of domain, we first define a degree function $\vartheta: R^\times \rightarrow \mathbb{N}$ with such two constraints: (1) $\vartheta(f)\leq \vartheta(fg)$ for all $f,g\in R^\times$. (2) for all $f,g\in R$ with $f\in R^\times$, there…
hxhxhx88
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Failure of group definition with weaker axioms

In "A First Course in Abstract Algebra", Edition 7, p.43, Fraleigh writes that It is possible to give axioms for a group $\left$ that seem at first glance to be weaker, namely: The binary operation $*$ on $G$ is associative. There…
Yiyuan Lee
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Number of $60$th primitive roots of $-1$

How many elements does the set $$\{z\in \mathbb{C}\mid z^{60}= -1; z^k \neq -1\text{ for } 0\lt k< 60\}$$ have? $1.\quad24$ $2.\quad30$ $3.\quad32$ $4.\quad45$ I assumed that set consists of elements of order $120$ (as $(-1)^{2}=1$) i.e no …
user118494
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Example of a ring without unity that has a subring with unity?

I can't think of a ring without unity that has a subring with unity. There must be some element in the parent ring that doesn't work with the subring's identity, but I'm struggling to see how that would be possible. Any suggestions?
bkaiser
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Ring Homomorphisms from $\mathbb Z_{20} \to \mathbb Z_{30}$

We need to find all ring homomorphisms from $\mathbb Z_{20} \to \mathbb Z_{30} $ ; I read its solution somewhere which states that : $R : \mathbb Z_{20} \to \mathbb Z_{30}$ defined by $R(x) = ax$ , $a$ belongs to $\mathbb Z_{30}$ is a ring…
User9523
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What does it mean "unique" for this author?

I'm studying Hungerford's Abstract Algebra book. I would like to know what the author means by "unique" in this theorem: The orders count? I mean the element $g\in G$ such that $g=a_{i_1}a_{i_2}=a_{i_2}a_{i_1}$ is considered unique? I'm asking…
user42912
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Showing that every field is an integral domain.

The proof I have starts of with $\;xy=0\;$ in a field. Then $x^{-1}$ exists because it is a field. Then $x^{-1} xy=x^{-1} 0$. Therefore $y=0$. But surely if an integral domain can not have any zero divisors, how can we end the proof by saying…
user204450
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Proof of $\mathbb Z/n\mathbb Z\bigotimes_{\mathbb Z}\mathbb Z/m\mathbb Z \cong Hom(\mathbb Z/n\mathbb Z, \mathbb Z/m\mathbb Z)$

How can we prove that $\DeclareMathOperator\Hom{Hom}\mathbb Z/n\mathbb Z\bigotimes_{\mathbb Z}\mathbb Z/m\mathbb Z \cong \Hom(\mathbb Z/n\mathbb Z, \mathbb Z/m\mathbb Z)$ without using the fact that $\mathbb Z/n\mathbb Z\bigotimes_{\mathbb Z}\mathbb…
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XOR is commutative, associative, and its own inverse. Are there any other such functions?

In particular, I was musing on this trick for swapping two values in a program without allocating any new variables. Wikipedia proves its correctness, and the proof picqued my curiosity. It relies on the following four properties of…
Matchu
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Difference between external and internal direct product?

What is the difference between external and internal direct product ?? I think both of them boil down to the same thing.
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Given fields $K\subseteq L$, why does $f,g$ relatively prime in $K[x]$ imply relatively prime in $L[x]$?

$K,L$ are fields, $K\subseteq L$. $f,g \in K[x]$. Suppose that $f,g$ are relatively prime as elements of $K[x]$. Prove they remain relatively prime in $L[x]$. I've tried everything I can think of. I feel like working with the contrapositive may be…
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Coset and ideal

Up to this moment I still have fuzzy ideas about some definitions in Abstract Algebra, especially about the difference between coset and ideal. Perhaps this question is a dumb one for you but I know precise definition plays a very crucial role in…
A.Magnus
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Preservation of ideals in localized rings

Refer to localization of rings in Lang's Algebra p. 108. In particular, let $A$ be a commutative ring, $S$ a subset of $A$ that is a submonoid of the multiplicative monoid structure of $A$, containing the identity. Then $S^{-1}A$ is the set of…
Manos
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