For questions about modules over rings, concerning either their properties in general or regarding specific cases.

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1answer
61 views

Composite function of $R$-module

Let $M$, $N$, $P$ be $R$-module and functions $f:M\rightarrow N$, $g:M\rightarrow N$, $h:N\rightarrow P$, $k:P\rightarrow M$, which need not be homomorphisms. Define $f+g:M\rightarrow N$ by ...
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0answers
24 views

Confused with a terminology used in a book Group Representation Vol 1 Part A by G Karpilovsky

I am confused with a terminology used in a book Group Representation Vol 1 Part A by G Karpilovsky (Chapter 10, page 441). Which is as follows: Let $A=\oplus_{g \in G} A_g$ be a $G$-graded ...
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1answer
28 views

$H^1(G,A)\rightarrow H^1(H,A)$ is onto

Note that $H^1(G,A)\rightarrow H^1(H,A)^{G/H}$ where $A$ is $G$-module and $H$ is a subgroup in $G$ But I suspect that ${\rm Res}\ : \ H^1(G,A)\rightarrow H^1(H,A)$ may be onto since $ f\in C^n ...
1
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1answer
57 views

Why is axiom of choice needed? (Equivalent conditions for Noetherian)

Let $R$ be a commutative ring with $1$ and let $M$ be a left $R$-module. On page 458 of Dummit and Foote's Algebra, 3rd edition, they show that $M$ is Noetherian (i.e. satisfies A.C.C. on submodules) ...
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1answer
31 views

Proving a submodule is Noetherian

I'm doing some independent study with a professor in Ring/Field theory (I'm an undergraduate) and I have been having a hell of a time wrapping my head around problems involving Noetherian rings and ...
1
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1answer
44 views

Matrices over PID

Let $R$ be a PID and $A,B\in\operatorname{M}_n(R)$ are $n\times n$ matrices such that $\det(A)\sim\det(B)\neq0$,i.e., the ideals generated by $\det(A)$ and $\det(B)$ are the same, does there exist ...
1
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1answer
58 views

A question about projective modules.

Suppose that we have a commutative ring $R$ with an idempotent $e$, and $M$ an $R$-module such that $Me$ is $Re$-projective. I am interested to know under which conditions this implies that $M$ is ...
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0answers
41 views

Gorenstein ring and projective module

I am new to this topic and would appreciate little explanation. Def: A commutative, unital ring $A$ is a cubic ring if $A$ is a free $\mathbb{Z}$-module of rank $3$. Def : A cubic ring $A$ is ...
2
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1answer
71 views

Self-injective ring but not semisimple?

It is well-known that if $K$ is a field, then $K[x]/(f(x))$ is a self-injective ring for any polynomial $f(x)$ in $K[x]$. On the other hand, we know that a ring $R$ being semisimple is equivalent to ...
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2answers
29 views

Injectivity of Inflation Homomorphism $H^1(G/K,A^K)\rightarrow H^1(G,A)$

I want to prove that injectivity of $inflation\ homomorphism$ $$0\rightarrow H^1(G/K,A^K)\rightarrow H^1(G,A)$$ where $K$ is normal. Proof : (a) If $x\in A^K$ then $gH\cdot x:= gx$ This is ...
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0answers
55 views

If $R$ is a integral domain then $TM$ is torsion module

Let $R$ be an integral domain. An $R$-module $M$ is torsion if $TM = M$. Prove that $TM$ is torsion. I'm confused with this exercise, i need to prove that $TTM = TM$ but we know that $TM$ is a ...
1
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1answer
40 views

If the factor of a finitely generated module is free then submodule is also finitely generated

All rings are commutative, associative and with 1. Consider short exact sequence $0 \rightarrow M' \rightarrow M \rightarrow M'' \rightarrow 0$ of $R$-modules. How to show that if $M$ is finitely ...
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0answers
23 views

$ A\rightarrow M^G_H(A)$ induces ${\rm Res} : H^n(G,A)\rightarrow H^n(H,A)$

I want to prove the following : $A$ is $G$-module. Then prove that $$ A\rightarrow M^G_H(A)$$ induce a restriction map $$ {\rm Res}\ : H^n(G,A) \rightarrow H^n(G,M^G_H(A)) =_\text{Shapiro} ...
1
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1answer
75 views

Complement but not direct summand; Lam, Lectures on Modules and Rings, Example 6.17(5)

Let $S$ be a submodule of an $R$-module $M$. A submodule $C⊆M$ is said to be a complement to $S$ (in $M$) if $C$ is maximal with respect to the property that $C∩S=0$. (This does exist by Zorn Lemma.) ...
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0answers
17 views

Example of syzygies not arising from polynomial rings

The Wikipedia page on syzygies describes a syzygy in general as a relation between the generators of a module. However, I find it difficult to find examples of syzygies that does not involve ...
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2answers
122 views

The Freyd-Mitchell Embedding Theorem and projective (injective) objects

Given a small abelian category $\mathcal{A}$, the Freyd-Mitchell Embedding Theorem gives me a fully faithful exact functor $F:\mathcal{A}\rightarrow R$-$\mathsf{Mod}$, for some unital ring $R$, so ...
0
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1answer
16 views

End(V) and End(V)xEnd(V) are isomorphic

Let R=End(V) be the ring of all linear endomorphisms of an infinite dimension complex vector space V with countable basis $\{e_{1},e_{2},...\}$ . Prove that R and RxR are isomorphic as left R-modules. ...
8
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1answer
171 views

Recovering free modules from their projective limit

Let $\dotsc A_2 \to A_1 \to A_0$ be a sequence of surjective homomorphisms of commutative rings. Consider the projective limit $\varprojlim_i A_i$. If $S$ is an (infinite) set, then $\varprojlim_i ...
0
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1answer
33 views

How to show $\alpha=i_L$ and other equalities without using isomorphism?

This is a question of Commutative Algebra.It was given in my class.I was able to solve it to some extend.But I have some doubts. Please help me. Thnx in advance. $A$ is a commutative ring with ...
0
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1answer
76 views

Intersection of all associated primes

Given $(R,m)$, a Noetherian local ring, and $M$ a nonzero $R$-module. I was wondering if there is a way to describe the elements of $\displaystyle\bigcap_{P\in Ass_RM} P$. In particular, when $M$ is ...
2
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1answer
40 views

Localization of a finitely generated module is trivial iff its annihilator is nontrivial

I have a problem on Atiyah and MacDonald's commutative algebra book, the exercise 3.1: Let $S$ be a multiplicatively closed subset of a ring $A$ and $M$ a finitely generated $A$ - module. ...
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0answers
42 views

Reference about Tensor Product

do you know some reference about tensor product of modules, with all elementary properties are proved ?? I want something a bit more explicit than "Commutative Algebra" of Atiyah and Macdonald. ...
1
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1answer
26 views

A uniform module as an intersection

Let $R$ be a semiprime, nonsingular ring with finite Goldie dimension u.dim $R_R$. (Nonsingularity means here that $Z(R_R)=0$, where $Z(R_R)$ is the set of elements $x$ of $R$ with $ann(x)$ is ...
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0answers
9 views

Does projectivity over the base ring imply projectivity over the extension?

Suppose $R\to S$ is a ring homomorphism. Let $M$ be an $S$-module. Suppose $M$ is projective as an $R$-module. Is $M$ projective as an $S$-module? If not, what additional hypothesis would suffice (for ...
2
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0answers
27 views

on basic theory over $\mathbb{Z}_4$-linear codes

I am a bit confused about this example in Huffman and Pless's Fundamentals in Error-Correcting Codes. This can be found in Chapter 12: Let $\mathcal{C}$ be a nonempty subset of $\mathbb{Z}{}^4_4$ ...
0
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1answer
21 views

Characterization of Zero in a module

Let $M$ be an $R-module$ over a ring $R$. Obviously, $0 \cdot m = 0, \forall m \in M$. Is it also true that $x \cdot m = 0, \forall m \in M$ if and only if $x = 0$?
2
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1answer
29 views

If $I$ and $J$ are distinct maximal ideals of $R$, any module homomorphism $R/I\to R/J$ is zero.

I am not sure how to deal with the following problem: $I$ and $J$ are distinct maximal ideals in a commutative ring $R$ and if $\phi$ is a modules homomorphism from $R$-module $R/I$ to $R$-module ...
0
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2answers
45 views

The difference between the free $A$-module $A^{(M\times N)}$, and $M\times N$

Let $M$ and $N$ be two $A$-modules, where $A$ is a commutative ring. What is the difference between the free $A$-module $A^{(M\times N)}$, and $M\times N$? In Atiyah-Macdonald, both of these seem to ...
4
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3answers
97 views

Why do we define modules to be over *abelian* groups?

We define a module to be an abelian group $M$ together with a ring action $R \times M \to M$ that satisfies certain properties. Q: Why do we require that $M$ is abelian? I know that modules ...
0
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0answers
63 views

Algebraic K-theory: Categories of modules and their equivalences

I'm currently preparing a presentation on the second chapter, called "Categories of modules and their equivalences", in Algebraic K-theory by Hyman Bass. I have a VERY elementary understanding of ...
0
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1answer
36 views

Proof of $0\rightarrow A^G\stackrel{f}\rightarrow B^G\stackrel{h}\rightarrow C^G\rightarrow 0$

When we have a $G$-modules exact sequence $0\rightarrow A\stackrel{f}\rightarrow B\stackrel{h}\rightarrow C\rightarrow 0$ we have a $G$-modules exact sequence $0\rightarrow ...
0
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2answers
60 views

tensor an exact sequence

Let $R$ be a ring, $M$ an $R$-module, and $I$ an ideal of $R$. Then given the exact sequence: $$0\rightarrow I\rightarrow R\rightarrow R/I\rightarrow0$$ when is $M$ flat. By some theorem, $M$ is ...
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2answers
46 views

Show that the homomorphism $\lambda: k[X] \to End_k(V) : p \mapsto p(A)$ corresponding to the $k[X]$-module strucutre of $V$ has a nontrivial kernel.

$\DeclareMathOperator{\End}{End}$ I'm trying to show that: Show that the homomorphism $\lambda: k[X] \to \End_k(V) : p \mapsto p(A)$ corresponding to the $k[X]$-module strucutre of $V$ as in (see ...
0
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1answer
37 views

Can the integral group ring construction be extended to groupoids in such a way that it provides a functor from Groupoids to Rings?

If $G$ is a group, and $R$ is a ring, one can construct the group ring $R[G]$, which is a free $R$-module with basis $G$, and with multiplication coming from the original multiplication of $G$. ...
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0answers
26 views

What maps of $k$-algebras $A\to B$ induce finite maps $\mathrm{Ext}_B^*(k,k)\to\mathrm{Ext}_A^*(k,k)$?

Let $k$ be an algebraically closed field, and let $A$ and $B$ be finitely generated $k$-algebras. A map $\varphi:A\to B$ of $k$-algebras induces a map ...
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0answers
21 views

Left and right module on the cohomology of a sheaf

Let $X$ a topological space, say a complex variety, and $\mathbb{C}_X$ its constant sheaf. $\mathcal{D}(X)$ is the derived category of sheaves of $\mathbb{C}_X$-modules. Let $F^\bullet\in ...
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0answers
45 views

Proof of ${\rm Tor}\ (A,B)={\rm Tor}\ (B,A)$

Problem : I want to prove that ${\rm Tor}$ is symmetric where $1\in R$ Definition : Note that if $B$ has a projective resolution $$ P_n\rightarrow_{d_n}\cdots P_0\rightarrow B\rightarrow 0 $$ By ...
0
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1answer
69 views

Proof of the theorem: Any $R$-module is contained in an $R$-injective module

Note that this an exercise in Foote and Dummit's Algebra. Problem: Let $1\in R$. Any $R$-module $M$ is contained in an $R$-injective module. Definition : $R$-module $Q$ is injective if ...
1
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1answer
61 views

Pushout,modules,short exact sequences

if we have this diagram in modules $$\begin{array}{ccccccccc} & & 0 & & 0 & & & & \\ & & \downarrow & & \downarrow & & & & \\ 0 ...
6
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0answers
69 views

Symmetric Algebra and Extension of Scalars

I am reading Gortz and Wedhorn's Algebraic Geometry. In their section on the symmetric algebra they explain the adjoint situation $$ \mathrm{Sym}_A \dashv i_A\colon \mathrm{Alg}(A)\to ...
1
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1answer
71 views

Failure of the Krull-Schmidt Theorem?

Theorem 1.19 of Representation Theory of Finite Groups: Algebra and Arithmetic is the Krull-Schmidt theorem, which I screenshotted and uploaded it here, I don't have any problem with this theorem and ...
0
votes
1answer
82 views

Does every free $R$-module have a maximal proper submodule?

Let $R$ be a commutative ring with $1$. We know that every finitely generated $R$-module has a maximal proper submodule. Is it true for any free $R$-module? In particular, can we do the following: ...
1
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1answer
30 views

Extension of an $R$-homomorphism as a sum

I want to solve this problem: Let $M$ be an injective module over a ring $R$ with identity, and $f$, $g$, $h$ are elements of the ring of $R$-endomorphisms $\operatorname{End}(M)$ such that $\ker ...
7
votes
2answers
104 views

Winning strategies in multidimensional tic-tac-toe

This question is a result of having too much free time years ago during military service. One of the many pastimes was playing tic-tac-toe in varying grid sizes and dimensions, and it lead me to a ...
-1
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3answers
80 views

Annihilator of annihilator of annihilator of a submodule

Let $R$ be a commutative ring and M be an $R$-module. The question is whether for every submodule $N$ of $M$ the equality ...
0
votes
1answer
37 views

A direct limit of pullbacks

Let an $R$-module $C$ be a direct limit of finitely presented $R$-modules $C_i$, and we have a short exact sequence as follows: $$0→A↪B\stackrel{\pi}→C→0.\qquad (S)$$ From each $C_i$ to the direct ...
1
vote
1answer
51 views

Example of a module such that every proper submodule is finitely generated but the module is not.

Let $R$ be a ring with 1 and $M$ an $R$-module. What is an example such that $M$ is infinitely generated but every proper submodule is finitely generated.
0
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0answers
50 views

Bass numbers of minimax modules are finite?

Let $R$ be a commutative Noetherian ring, and $M$ be a minimax $R$-module. Are the Bass numbers of $M$ are finite? (An $R$-module $M$ is called minimax, if there is a finite submodule $N$ of $M$, such ...
0
votes
1answer
88 views

Modules with finite support in $\mathrm{Max}(R)$

Let $R$ be a commutative Noetherian ring, and $M$ be an $R$-module. Is the following statement true? If $\mathrm{Supp}_R(M)$ (support of $M$) is a finite subset of $\mathrm{Max}(R)$ (the set of all ...
3
votes
1answer
47 views

Injective hull of $\mathbb{ Z}_n$ [duplicate]

What is the injective hull of $\mathbb Z_n$? I know that in case $n=p$ is prime, the injective hull would be isomorphic to $\mathbb Z_{p^∞}$, but in general case, I have no idea. Can anyone be of ...