# Tagged Questions

**11**

votes

**2**answers

1k views

### Example of a Group which has $\text{SL}_{n}(\mathbb{Z})$ as the automorphism group

For the past one week, I have been trying to learn more about Automorphism groups of different groups. Very recently one of my friend asked this question to me:
What is the Autmorphism group of ...

**1**

vote

**1**answer

276 views

### Defect groups and subgroups

I have asked this question on Stack Exchange but had no response; it's been bugging me for a few days. I am struggling to see how to apply Mackey's theorem to prove a certain Lemma in Local ...

**5**

votes

**2**answers

2k views

### Example of an infinite abelian but non-cyclic group whose automorphism group is cyclic.

Can anyone give me an example of:
An infinite abelian but non-cyclic group whose automorphism group is cyclic.

**14**

votes

**2**answers

412 views

### Formally undecidable problems on finitely presented quandles

In the literature, one sometimes sees the claim that finitely presented quandles (in particular, knot quandles) are "hard to deal with". Hence, a great deal of effort has gone into studying finite ...

**4**

votes

**2**answers

6k views

### An image of the hierarchy of algebraic structures

Hello! Does anybody know an image of a graph featuring the hierarchy of algebraic structures? Something rather complete.
So far I've found similar images describing the hierarchies of ...

**2**

votes

**3**answers

603 views

### Are these Two Definitions of Quadratic Form (Algebraic, Topological) Related to Each Other?

Hi, All:
I am trying to see if there is a nice relation between two different definitions of quadratic form q; a topological definition $q_T$, and an algebraic definition $q_A$, and, if there is, how ...

**2**

votes

**2**answers

542 views

### Difference between orthogonal form and seminormal form

Frequently in the literature on Hecke algebras for the symmetric group and their generalisations, one encounters references to Young's seminormal form and Young's orthogonal form. I have a good ...

**0**

votes

**3**answers

4k views

### Suggestions for a good abstract algebra book [closed]

i am early undergraduate looking for a good textbook gor algebra, i don't want a too wordy book@@

**7**

votes

**2**answers

2k views

### fgf = f, gfg = g, fg not necessarily identity, what was that called?

A very simple question, I just totally forgot how it was called, and google is not helping.
There's a pair of functions $f:X\to Y$, $g:Y\to X$.
$fgf = f$, $gfg = g$, but $fg$ and $gf$ don't need to ...

**0**

votes

**2**answers

319 views

### Aut(G) acting transitively on a finite group [closed]

How Aut(G) acting transitively on a finite group G^* can lead G to be elementary abelian group? G^*=G-{1}.

**0**

votes

**1**answer

427 views

### Number of ideals in primary decomposition

Dear friends,
I have the following question:
Let K be an algebraic number field and [K:Q]=n. Let O_K be a full ring of integers of K.
Assume that O \subset O_K is a subring such that rank of O over Z ...

**0**

votes

**1**answer

628 views

### Groups GLn(F) and PSLn(F)

As J.S.Rose noted in his book "A Course on Group Theory" : There is a section of GLn(F) which is isomorphic to PSLn(F), n≥1, F is a field"?. I ask that "What can this section be?"

**3**

votes

**3**answers

596 views

### About solvable groups

Is it possible for a group (non-simple and non-abelian) that solvability of all of its proper subgroups leads the whole group to be solvable?

**0**

votes

**2**answers

217 views

### The X-series (for groups)

It goes without saying that the name in the title tentatively refers to a series whose name one does not know yet and probably in the future I may come with a post titled "The x-sequence" or "The ...

**1**

vote

**1**answer

416 views

### Unimodular column property

Hi, I know that if $R$ is a ring such that every projective $R$-module finitely generated is free then $R$ has the unimodular column property.
I would like to know if there is a ring $R$ that doesn't ...

**7**

votes

**1**answer

739 views

### Direct sum of injective modules over non-Noetherian rings

Hi. I know, by the Bass-Papp theorem, that if every direct sum of injective $R$-modules is injective then $R$ is Noetherian. I would like to know if there exists a direct sum of injective $R$-modules ...

**7**

votes

**5**answers

2k views

### motivation of filtered colimits

I am trying to move in categorical algebra beyond the basics. A Lawvere theory L is a small category with finite products. (I know that there also is a functor $(skeleton(FinSet))^{op}\to L$, which ...

**2**

votes

**2**answers

362 views

### Related to fractional ideals

$K$ a field, $A\subset K$ a subring, $M\subset K$ an $A$-submodule. Define
$$(A:_{K}M):= \lbrace s\in K|sM\subset A\rbrace$$
Then it is easy to see that
$$M\subset A\Longleftrightarrow A\subset ...

**6**

votes

**3**answers

871 views

### Does “finitely presented” mean “always finitely presented”, considered in general

I'm wondering about the question, "If we have a finitely presented __, is it necessarily finitely presented with respect to any finite generating set for it?" I know this is true for groups and for ...

**14**

votes

**4**answers

3k views

### Unique factorization domains

Let $c$ be an integer, not necessarily positive and $|c|$ not a square. Let $\mathbb{Z}[\sqrt{c}]$ be the set of complex numbers $$a+b\sqrt{c}, a, b\in \mathbb{Z},$$
which form a subring of the ring ...

**0**

votes

**1**answer

141 views

### Projectively splitting module

Is there a name for such class of modules $M$ such that $M\rightarrow N\rightarrow 0$ splits for every $N$ ?

**-2**

votes

**2**answers

621 views

### Unit ideal in non-commutative rings [closed]

In a non-commutative ring (with identity), is it possible for an element which does not possess left or right inverses to generate the entire ring? i.e. $(r)=R$, where (r) is the two-sided ideal ...

**0**

votes

**2**answers

1k views

### Union of two proper subgroups [closed]

When can a group be written as the set-theoretic union of its proper subgroups?

**3**

votes

**1**answer

1k views

### Bounds for zeros of polynomials with only real zeros

The formula in
http://de.wikipedia.org/wiki/Diskussion:Nullstelle#References
gives upper and lower bounds x1 and x2 for the roots of a polynomial all of whose roots are real.
Where can I find a ...

**16**

votes

**2**answers

2k views

### Does any textbook take this approach to the isomorphism theorems?

Below, I present an outline of a proof of the first isomorphism theorem for groups. This is how I usually think of the first isomorphism theorem for ______, but groups will get the points across. My ...

**5**

votes

**1**answer

372 views

### What is the Schouten bracket for the Chevalley-Eilenberg complex with coefficients in a nontrivial module?

Let $\mathfrak g$ be a Lie algebra. The Chevalley-Eilenberg complex is defined to be $\wedge^* \mathfrak g$ with differential $d\colon \wedge^* \mathfrak g\to \wedge^{*-1}\mathfrak g$ defined by ...

**8**

votes

**4**answers

647 views

### Groups and rings which are not sets

An algebraic structure such as a group, ring, field, etc. is usually defined to be a set with some operations satisfying certain properties. I am curious what, if anything, goes wrong when the ...

**15**

votes

**1**answer

847 views

### Is Dependent Choice equivalent to the statement that every PID is factorial?

In this question, it was asked if AC is needed in the proof of the well-known fact that every principal ideal domain is factorial. As KConrad and Joel David Hamkins have pointed out, only DC, the ...

**78**

votes

**18**answers

7k views

### How do you decide whether a question in abstract algebra is worth studying?

Dear MO-community, I am not sure how mature my view on this is and I might say some things that are controversial. I welcome contradicting views. In any case, I find it important to clarify this in my ...

**2**

votes

**7**answers

2k views

### How to tell if two random polynomials are identical

Let t be a positive real number. Let P(x) and Q(x) be two random polynomials with integer coefficients. If P(t) = Q(t), then what is the probability that P(x) is not identical to Q(x)?
Will it make a ...

**6**

votes

**1**answer

2k views

### Non-trivial consequences of Baer's theorem and Lucchini's theorem in subnormality theory

There are a couple of beautiful results in finite group theory that look trivial, at least on a first glance, but require non-trivial facts to prove. I am basically interested in whether these results ...

**2**

votes

**2**answers

265 views

### Admissible finite groups

I want to know when an abelian group of even order is admissible (or has a complete map)? And when a nonabelian group of even order is admissible (or has a complete map)?
Thanks.

**2**

votes

**1**answer

668 views

### Rank of a linear combination of quadratic forms

Suppose we have a set of quadratic forms $Q_i (x_1, \dots, x_n)$ for $1 \leq i \leq k$ in $n$ variables, defined over $\mathbb{R}$. We suppose these are 'collectively nondegenerate' in the sense that ...

**25**

votes

**7**answers

3k views

### Consequences of not requiring ring homomorphisms to be unital?

As defined in many modern algebra books, a homomorphism of unital rings must preserve the unit elements: $f(1_R)=1_S$. But there has been a minority who do not require this, one prominent example ...

**3**

votes

**5**answers

563 views

### “Convex Optimization” over varying-dimension vector space?

For all instances of Convex Optimization I know of, the dimension of the vector space is defined beforehand.
Is there an area of mathematics that deals with "convex optimization" of varying-dimension ...

**4**

votes

**2**answers

2k views

### Sylow's theorem 3rd Proof Page 96 I.N.Herstein

I was just going through the 3rd Proof of Sylow's theorem given in the "Topics In Algebra" Book by I.N. Herstein. It looked very interesting and i really liked its Philosophy. My question what is its ...

**4**

votes

**5**answers

2k views

### Indeterminate “x” in Abstract Algebra/Ring Theory [closed]

How do you interpret the indeterminate "x" in ring theory from the set theory viewpoint? How do you write down R[x] as a set? Is it appropriate/correct to just say that
$R[x] = \{ f: R \to R | ...

**1**

vote

**2**answers

1k views

### Extension problem

As I understand, if $0\rightarrow A\rightarrow X\rightarrow B\rightarrow 0$ is a short exact sequence of abelian groups, $\mbox{Ext }_{\mathbb{Z}}^{1}(B,A)$ gives all the isomorphism classes of what ...

**12**

votes

**4**answers

2k views

### Linearly independent subsets of a free module

Is it true that the cardinality of every maximal linearly independent subset of a finitely generated free module $A^{n}$ is equal to $n$ (not just at most $n$, but in fact $n$)? Here $A$ is a nonzero ...

**21**

votes

**6**answers

2k views

### How fast are a ruler and compass?

This may be more of a recreational mathematics question than a research question, but I have wondered about it for a while. I hope it is not inappropriate for MO.
Consider the standard assumptions ...

**1**

vote

**1**answer

1k views

### Cardinality of symmetric group [duplicate]

Possible Duplicate:
Cardinality of the permutations of an infinite set
Why does the symmetric group on an infinite set X have the cardinality of the power set ${\cal P}(X)$?

**29**

votes

**3**answers

3k views

### Feit-Thompson Theorem: The Odd Order Paper

For reference, the Feit-Thompson Theorem states that every finite group of odd order is necessarily solvable. Equivalently, the theorem states that there exist no non-abelian finite simple groups of ...

**1**

vote

**2**answers

459 views

### Understanding the modules of semiprimitive rings

As far as I understand, a semiprimitive ring can be fully 'explored' by its simple modules, in the sense that a semiprimitive ring is the subdirect product of its simple modules (for brevity, I'll use ...

**2**

votes

**2**answers

275 views

### Explicit representations of finite fields

An old question that occurred to me again recently: are there any explicit formula known for sequences of irreducible polynomials $g_{p^n}(X)$ in $Z/pZ[X]$ such that for the finite field with $p^n$ ...

**6**

votes

**2**answers

4k views

### Is there a way to simplify block Cholesky decomposition If you already have decomposed the sub matrices along the leading diagonal?

Lets say we have a block matrix $ M =\left( \begin{array}{ccc}
A & B\\\\
B^{*} & C \end{array} \right)$ where M is positive definite. (A, and C are also pos def)
There is a formula for ...

**8**

votes

**10**answers

8k views

### Learning Algebra & Group Theory on my own [closed]

I'm learning Algebra & Group Theory, casually, on my own. Professionally, I'm a computer consultant, with a growing interest in the mathematical and theoretical aspects. I've been amazed with ...

**7**

votes

**2**answers

752 views

### The word “torsion” and its connection to geometry and homology

In an $R$-module $M$, an element $m \in M$ is said to be torsion if $am = 0$ for some $a \in R$ with $a \neq 0$.
Also, for a non-orientable (closed) surface such as the projective plane or the Klein ...

**10**

votes

**5**answers

6k views

### Does category theory help understanding abstract algebra?

I'm studying category theory now as a "scientific initiation" (a program in Brazil where you study some subjects not commonly seen by a undergrad), but as I've never studied abstract algebra before, ...

**1**

vote

**3**answers

2k views

### terminology about ring/algebra in abstract algebra and measure theory

Both in abstract algebra and measure theory is there term ring/algebra, but their definition are different and we can not deduce one from the other, the only requirement in definition they share is ...

**25**

votes

**9**answers

5k views

### What are the reasons for considering rings without identity?

I think a major reason is because Lie algebras don't have an identity, but I'm not really sure.