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

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16 views

Let $M$ be an $R-$module and $x\in M\setminus\left\{ 0\right\} $. Prove that there exists a left ideal of $R$, say $I$ such that $Rx\cong R/I $.

Let $M$ be an $R-$module and $x\in M\setminus\left\{ 0\right\} $. Prove that there exists a left ideal of $R$, say $I$ such that $Rx\cong R/I $. Help me some hints. Thank you in advance.
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2answers
33 views

IM is a submodule of M

In the fisrt answer of this question where $R$ is a ring, $I$ is a left ideal of $R$ and $M$ an $R$-module; I don't know why can't I see that $IM$ is closed under addition. If we take two elements $x$ ...
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1answer
25 views

Semisimplicity of the ring $\mathbb Z_n$

I am being asked to figure out when $\mathbb Z_n$ is a semisimple ring. It is clear to me that if $n$ is prime then $\mathbb Z_n$ is simple, which implies it is semisimple. If $n=p_1...p_n$ is a ...
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1answer
24 views

Equivalence of categorical coproduct proof

quiLet $C$ be an abelian category and {$X_1$,...,$X_n$} a finite family of objects in that category. ( $X$,($M_i$: $X_i$$\to$ $X$) where $i_1$=1,....n a coproduct of the finite family if and only if ...
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2answers
32 views

Prove that an exact sequence splits

Let $0 \to r\mathbb{Z}_n \to \mathbb{Z}_n \to s\mathbb{Z}_n \to 0$ where n =rs an exact sequence of $\mathbb Z$ modules the how can I prove the sequence splits if and only if $(r,s)=1$ The only thing ...
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0answers
21 views

Simple problem about morphism in abelian categories

$f$ : $X\to$ $Y$ and $g$ : $Y\to$$Z$ a sequence in abelian categories. Show that if $gf$=$0$ if and only if exist a monomorphism $h$:$Im(f)$ $\to$ $Ker(g)$ such $kh$=$j$, where $j$:$Im(f)$$\to$ $Y$ ...
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1answer
42 views

Finite abelian groups (application of structure theorem)

Problem Find all finite abelian groups that simultaneously have exactly $7$ elements of order $2$, exactly8 elements of order $3$, exactly $8$ elements of order $4$, at least an element of order ...
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1answer
45 views

Let $R=M_n(D)$, $D$ is a division ring. Prove that every $R-$simple module is isomorphic to each other. [duplicate]

Let $R=M_n(D)$, $D$ is a division ring. Prove that every $R-$simple module is isomorphic to each other. I need some hints to prove it. Thank you very much.
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1answer
28 views

Exact sequence of modules exercise

Show that if $$0 \rightarrow M_1 \xrightarrow{f} M_2 \xrightarrow{g} M_3$$ is an exact sequence of $R$-modules, then for all $R$-module $$0 \rightarrow \operatorname{Hom}_R(M,M_1) \xrightarrow{f_*} ...
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1answer
54 views

Constructing pullback and pushout problem

i) Let $p$ be a prime and $f: \Bbb Z \rightarrow \Bbb Z_p$ and $g: \Bbb Z_{p^2}\rightarrow\Bbb Z_p$ be the canonical epimorphism. Show that the pullback of $f$ and $g$ is isomorphic to $\Bbb Z ...
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1answer
64 views

Are two bimodules isomomorphic as left and right modules also isomorphic as bimodules?

let R be a commutative ring, and M, N two bimodules over R, such that there exists f : M -> N an isomorphism of left R-modules, and g : M -> N an isomorphism of right R-modules. Then are M and N ...
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1answer
36 views

Exact sequence and Noetherian modules

Let $R$ be a ring, $X,Y,Z$ and $T$ four $R-$ modules such that there exists a short exact sequence $$0 \rightarrow X \xrightarrow{f_1} Y \xrightarrow{f_2} Z \xrightarrow{f_3} T \rightarrow 0 $$ Prove ...
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1answer
27 views

Finitely generated modules over a PIR (structure theorem application)

Let $R:=\mathbb R[x]/\langle (x^2+1)^2 \rangle$ and $J:=\langle x^2+1 \rangle \lhd R$. Prove that every finitely generated $R$-module is isomorphic to $R^m \oplus (R/J)^n$ for a unique pair of non ...
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2answers
50 views

If $M/N$ is isomorphic to $M/N'$, can we conclude that $N$ is isomorphic to $N'$?

Let $N$ and $N'$ be submodules of $R-$module $M$. If $M/N$ is isomorphic to $M/N'$, can we conclude that $N$ is isomorphic to $N'$? I believe the answer is no but I can't find a counter example. Help ...
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1answer
98 views

Atiyah-MacDonald Ch. 4 exercise 20: what's the module analogue of $\sqrt{\mathfrak{a}+\mathfrak{b}} = \sqrt{\sqrt{\mathfrak{a}}+\sqrt{\mathfrak{b}}}$?

Atiyah-MacDonald exercises 20-23 in chapter 4 develop a theory of primary decomposition for modules, in analogy with the theory developed in the chapter for rings. Exercise 20 begins with this ...
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31 views

Group presentation and Smith normal form

Problem Let $A=\langle a,b: a-3b=0,3a=3b \rangle$ and $B=\mathbb Z^3/S$ with $S=\{m \in \mathbb Z^3: m_1+2m_2+m_3=0, 5|m_3 \}$. Calculate $\operatorname{Hom}_{\mathbb Z}(A,B)$. My attempt at a ...
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1answer
21 views

Representatives of simple $\mathbb C[\mathbb D_3]$-modules (left modules)

Problem Let $\mathbb D_3$ be the symmetry group of the equilateral triangle. Give a complete list of the representatives of the simple left $\mathbb C[\mathbb D_3]$-modules. My attempt at a solution ...
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0answers
26 views

Let $p,q$ and $r$ be positive prime numbers. Determine the number of abelian groups of order $p^6q^3r$

Let $p,q$ and $r$ be positive prime numbers. Calculate the number of non isomorphic abelian groups of order $p^6q^3r$. I've tried to use the structure theorem. So we have $$G \cong \mathbb Z/\langle ...
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0answers
29 views

Characterize all $\mathbb Z[i]$ modules of order $21$ and $65$

I am trying to figure out how many $\mathbb Z[i]$ modules (up to isomorphisms) are of $21$ and $65$ elements. I've done a few similar exercises for finite abelian groups ($\mathbb Z$ modules) but I am ...
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0answers
18 views

Invariant factors of finite abelian group

Calculate the invariant factors of the group $G=\mathbb Z_{12} \oplus \mathbb Z_{21} \oplus \mathbb Z \oplus \mathbb Z \oplus \mathbb Z_{20} \oplus \mathbb Z_{9} \oplus \mathbb Z_7$. Applying the ...
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1answer
37 views

How can I visualize or understand a module in concrete terms?

As I understand it, a module is a ring action on an abelian group, usually a group that carries the operation of addition. I get this because I think of it as a set of elements where these abstract ...
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43 views

$G$ finite abelian group, $p$ prime that divides order of $G$

Problem Let $G$ be a finite abelian group and $p$ a positive prime that divides $|G|$. Show that the number of elements of order $p$ in $G$ is coprime with $p$. Let $|G|=p^nm$ with $n \geq 1$ and ...
2
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1answer
32 views

Is {0} a free module?

Is $\{0\}$ a free module (over any ring $R$) ? A free module is isomorphic to $R^n$, but is $n=0$ allowed? Alternatively, a free module is defined to have a set of linearly independent generating ...
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1answer
48 views

Is there a non-projective submodule of a free module?

Is there an example of a commutative ring $R$ with identity such that there exists a free $R$ module $M$ that has a non-projective submodule? I tried experimenting with modules over $\mathbb Z$ but ...
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1answer
34 views

Abelian group of order $p^2q^2$ ($p$,$q$ distinct primes) determine number of elements of order $pq$ and $pq^2$

Problem For each abelian group of order $p^2q^2$ determine the number of elements of order $pq$ and the number of elements of order $pq^2$ in $G$. By the structure theorem we have that $$(1) ...
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0answers
35 views

$I+J=R$ and $IJ$ is principal. prove that $I$ is a projective module.

let $I$ and $J$ be ideals of a commutative ring with identity $R$. assume that $I+J=R$ and that $IJ$ is principal. prove that $I$ is a projective module. I am familiar tiwh the theorem that states ...
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56 views

Semisimple ring and finitely generated modules exercise

Problem Let $R$ be a semisimple ring (i.e. $R$ is semisimple as a left $R$-module). Show that if $M,N$ and $P$ are finitely generated $R$-modules such that $M \oplus P \cong N \oplus P$, then ...
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22 views

Show that a heptagon is not constructible with a compass and straight edge.

I've so far worked out that we need to prove that we cant construct the angle $\frac{2\pi}7$, or that point on the unit circle $e^{i\frac{2\pi}7}$. What I dont understand is solving for the roots of ...
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1answer
91 views

non-examples for Krull-Schmidt-Azumaya

I am looking for some rings $A$ and finitely generated $A$-modules $M$, where the conclusion of the Krull-Schmidt-Azumaya Theorem does not hold for $M$, i.e. where $M$ can be written as direct sums of ...
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39 views

Additive exact functors preserve homology of modules

If $C$ is a chain complex of modules over a ring $R$ and $F: Mod_R \to Mod_S$ an additive exact functor then: If $F$ is covariant then $H_n(FC)\cong FH_n(C)$. Can anyone give my an idea or an ...
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1answer
97 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 ...
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1answer
77 views

Trouble showing flatness

Let $K$ be a field and $\pi: K[x]/(x^2) \to K$ be the ring homomorphism given by the valuation at $0$. I'm stuck in showing that $\pi^*(K)$ (the pullback) is not a flat module (over $K[x]/(x^2)$).
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1answer
36 views

Finitely generated module with free submodule $S$ and $M/S$ torsion free implies $M$ is free

Let $R$ be a PID and $M$ a module. Show the following: (i) If $M$ is finitely generated and $S$ is a free submodule with $M/S$ torsion free, then $M$ is free. (ii) If $M$ is torsion free ...
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1answer
74 views

A problem with tensor products

Let $K$ be a field, $R=K[x^2,x^3]$, $S=K[x]$, and consider $S$ as an $R$-module. Given $f: S \to R \oplus R$ so that $f:p \mapsto (x^3p,-x^2p)$, prove that $f\otimes 1: S \otimes_R S \to (R\oplus R) ...
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2answers
226 views

Quotient of a free $\mathbb{Z}$-module

I'm trying to find the quotient of a free $\mathbb{Z}$-module, but somehow I don't really find the right procedure on how to get the right quotient (nor have I found any sources). I've read What does ...
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1answer
29 views

Polynomial greatest common divisor algorithem

I look for an algorithm for Polynomial greatest common divisor. I saw this at Wikipedia but I didn't understand where is the algorithm. If you have other source for this algorithm, or you can write ...
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1answer
26 views

Module and Noetherian/Artinian Rings

I am trying to prove that: Every finitely generated $F$-module $M$ is both Noetherian and Artinian where $F$ is a field. For this I am looking at the submodules of $F$ and saying that they are in ...
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2answers
45 views

A non-projective module

Let a ring with identity $R$ be decomposed as $S_1⊕\cdots⊕S_n$ (as a right $R$-module), where $S_i=e_iR$ with $e_i$ nonzero idempotents of $R$ adding up to $1$. If $J$ is the Jacobson radical of ...
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1answer
73 views

If M is free with a finite basis then every basis of M over R is finite and has the same number of elements.

Stuck on a proof in my lecture notes. Proposition: Let $R$ be a commutative ring and let $M$ be an $R$-module. If $M$ is free with a finite basis then every basis of $M$ over $R$ is finite and has ...
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1answer
42 views

Equalities in the categories of modules

It is well-known that over a quasi-Frobenius ring $R$ any f.g. right module $M$ is reflexive, in the sense that whenever we take $M^*=Hom_R(M,R_R)$ as a left $R$-module, the modules $M$ and $M^{**}$ ...
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1answer
49 views

Finitely generated modules [duplicate]

Suppose that $R$ is a commutative ring and $M$ and $N$ are finitely generated $R-$modules. What we can say about $Hom_R (M,N)$? is it a finitely generated $R$-module?
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128 views

When is $\operatorname{gr}_I (M)$ finite?

When is $\operatorname{gr}_I (R)$ (I mean associated graded ring of $I$) finite? When is $\operatorname{gr}_I (M)$ finite? if 1) $R$ is non-Noetherian ring , 2) $R$ is Noetherian ring and $M$ ...
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2answers
52 views

Behavior of the annihilators of modules via monic and epic homomorphisms

Let $f:M\rightarrow N$ be an $R-$ homomorphism. Prove that if $f$ is monic, then $l_{R}\left(M\right)\supseteq l_{R}(N)$ , whereas if $f$ is epic, then $l_{R}(M)\subseteq l_{R}(N)$ . This is ...
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1answer
135 views

Do these facts about $\mathbb{Q}/\mathbb{Z}$ generalize to more general cases?

We know that $\mathbb{Q}/\mathbb{Z} $ is injective. Not only that $0 \to A \to B \to C \to 0$ is exact iff $0 \to Hom(C,\mathbb{Q}/\mathbb{Z}) \to Hom(B,\mathbb{Q}/\mathbb{Z})\to ...
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1answer
30 views

showing Module is simple

Given the following: let $C \subset \mathbb{H}$ be a subring of the real quarternion algebra such that it contains the center of $\mathbb{H}$ = $Z(\mathbb{H})$ Also C $\cong \mathbb{C}$ Then let R ...
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3answers
23 views

arithmetic modulo field with real numbers vector space

if i choose modulo3 to be the field, and the real numbers to be the vector space. how do i Multiplier vector in scalar? for example i take "4" from the vector space of real numbers and want to ...
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1answer
25 views

How to determine $Hom (M,M)$ for an irreducible $R$-module $M$?

More exactly, I'm considering $R$ to be a finite dimensional $\mathbb C$-algebra. For any $R$-module $M$, the $\mathbb C$ vector space $Hom_R(M,M)$ contains scalar multiplication and hence contains ...
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0answers
41 views

Equivalence of Modules

The category of modules over a ring can be viewed as an enriched version of an action of a monoid on a set (see nLab entry). Moreover, if $R$ is a commutative ring, the category of modules over it is ...
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1answer
31 views

Finding all $\mathbb{Z}$-submodules of $\mathbb{Z}$ with a complement

I am trying to find all the $\mathbb{Z}$-submodules of $\mathbb{Z}$ that have a complement Here with complement I mean: Def: Let $N \subset M$ be an $R$-Submodule. A submodule $N'$ is called ...
2
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1answer
12 views

homomorphism of product of modules

Given $(A_i)_{i\in I}$ and $B$ modules over a ring $R$, with $I$ infinite, $A_i\ne 0\;\forall i$ and $B\ne 0$, is it true that $$Hom_R(A_i,B)=0\;\forall i\implies Hom_R(\Pi_{i\in I} A_i, B)=0\quad?$$ ...