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## Hot answers tagged modules

10

Nakayama Lemma. Let $N$ be a finitely generated $R$-module, and $J\subseteq R$. Suppose that $J$ is closed under addition and multiplication and $JN=N$. Then there is $a\in J$ such that $(1+a)N=0$. (Here by $JN$ we denote the subset of linear combinations of $N$ with coefficients in $J$.) Cayley-Hamilton Theorem. Let $A$ be a commutative ring, $I$ an ...

10

No: For a local ring $(R,{\mathfrak m})$ with ${\mathfrak m}^2=0$ you have $\text{Hom}_R(R/{\mathfrak m},R)\cong\{x\in R\ |\ {\mathfrak m}x=0\}={\mathfrak m}$, so it suffices to choose $R$ such that ${\mathfrak m}$ is not finitely generated, e.g. $R := {\mathbb k}[x_1,x_2,\ldots]/(x_i^2, x_i x_j)$.

9

The simpler tensor product $\mathbb{Z}[i] \otimes \mathbb{Z}[i]$ is free abelian on the generators $1 \otimes 1, 1 \otimes i, i \otimes 1, i \otimes i$. As either a left or a right $\mathbb{Z}[i]$-module it is free on two generators $1 \otimes 1, i \otimes i$. This tensor product is the quotient of the above tensor product by the additional relation that $a ... 9 No, your argument isn't correct: if$1\otimes x=1\otimes y$then not necessarily$x=y$. I'd do this as follows: if$M$is a$\mathbb Z$-module, then$\mathbb Q\otimes_{\mathbb Z}M\simeq S^{-1}M$, where$S=\mathbb Z\setminus\{0\}$. The isomorphism is given by$\dfrac ab\otimes x\mapsto\dfrac{ax}{b}$. This way$1\otimes(1,1,\dots)$corresponds to the fraction ... 8 If$R=\mathbb{Z}[2i]$, then$\mathbb{Z}[i] \cong R[X]/I$, where$I=(X^2+1,2X-2i)$. So we have$\mathbb{Z}[i] \otimes_R R[X]/I \cong \mathbb{Z}[i][X]/I$. Substituting$Y=X-i$, this is$\mathbb{Z}[i][Y]/(Y^2+2iY,2Y) \cong \mathbb{Z}[i][Y]/(Y^2,2Y) \cong \mathbb{Z}[i] \oplus \mathbb{Z}[i]/2\mathbb{Z}[i]$. We can also see this isomorphism more directly by ... 7$\mathbb{Z}$-modules are precisely abelian groups. As every ring is an abelian group, it is a$\mathbb{Z}$-module. It is entirely possible to be a module over more than one ring. For example, if$M$is an$R$-module then it is also an$S$-module for any subring$S$of$R$(you seem to be interested in the case where$M = R$). Another example is given by ... 7 The only other examples of non IBN rings that I am aware of are built using Leavitt path algebras, and they do exactly what you want. It is possible to specify positive integers$n<m$such that$R^i\ncong R^j$for$(i,j)$less than$(n,m)$in the lexicographic order, but$R^n\cong R^m$as modules. (The$(m,n)$pair is called the 'module type' in the ... 7 This is not true in general. For example,$\mathbb{Q}\otimes_{\mathbb{Z}}(\mathbb{Z}/p\mathbb{Z}) \cong 0 \cong 0\otimes_{\mathbb{Z}}(\mathbb{Z}/p\mathbb{Z})$but$\mathbb{Q}$and$0$are not isomorphic as$\mathbb{Z}$-modules. 7 For question 2,$F\otimes_A -$has a left adjoint iff$F$is finitely generated, and the left adjoint is always exact. For if$(f_i)\in F^I$is an element of an infinite product of copies of$F$, then it is easy to see$(f_i)$is in the image of the canonical map$F\otimes_A A^I\to F^I$iff$\{f_i\}$is contained in a finitely generated submodule of$F$. ... 6 For every nonzero$x, y\in\mathbb{Q}$, there are nonzero integers$m, n$such that $$mx=ny$$ (where$mx, ny$are interpreted in the obvious way). Now, any homomorphism$f$between$(\mathbb{Q}, +)$and another structure$(G, *)$must preserve multiplication by integers: $$f(mx)=mf(x).$$ So if$(\mathbb{Q}, +)\cong(\mathbb{Q}^n, +)$, then$(\mathbb{Q}^n, +)$... 6 If$A$is any ring, then$A$and$M_n(A)$have equivalent categories of modules, and usually$A$and$M_n(A)$are not isomorphic. This is the simplest example of a Morita equivalence. 6 For the second question, the finite rings are precisely those for which every finitely generated module has finitely many submodules. For a ring$R$and$r\in R$, let$M_r$be the submodule of$R\oplus R$generated by$(1,r)$. Then$(1,r)$is the only element of$M_r$whose first coordinate is$1$, and so$M_r\neq M_s$for$r\neq s$, and so if$R$is ... 6 No. Let$G$be any non-zero abelian group. Then the obvious morphism$(0,G)\to (G,G)$is a monomorphism and epimorphism, but not an isomorphism, which can't happen in an abelian category. 6 Your idea is right: If$x$has finite order and your group homomorphism is$\phi$, then$\phi(x)$has an order that is less than or equal to that of$x$. This is true regardless of whether$\phi$is 1-1 or not. Thus you can't map onto elements of infinite order. When I read the original post, I misread that the indexing was over$p$! But of course, if you ... 6 The argument for modules generalizes trivially, once you phrase it properly. For any object$A$, there is an epimorphism$p:F\to A$from a free object$F$. If$A$is projective, then applying the definition of projectivity to$p$and the identity map$1:A\to A$, there exists a map$i:A\to F$such that$pi=1$. That is,$A$is a retract of$F$. (In the ... 6 No, for two reasons. First,$\frac{1}{q} m$must be a$q^{th}$root of$m$, but such a$q^{th}$root need not exist: for example take$M = \mathbb{Z}$. Second, if$m = qn$then$\frac{1}{q} m = \frac{1}{q} qn = n$, so$q^{th}$roots also need to be unique, and even when they exist in some abelian group they need not be unique: for example take$M = ...

6

This is never true unless $\mathfrak{m}=0$, i.e. unless $A$ is a field. Let $x\in\mathfrak{m}$, and consider the short exact sequence $0\to (x)\to A\to A/(x)\to 0$. If $A/\mathfrak{m}$ is flat, we can tensor it with the short exact sequence to get a short exact sequence $0\to (x)/\mathfrak{m}(x)\to A/\mathfrak{m}\to A/\mathfrak{m}\to 0$. Exactness of this ...

5

One way to proceed is to notice that $\mathbb Z_8$ is a local ring, so that its finitely generated projective modules are in fact free. Of course, this implies that finitely generated modules have at least $8$ elements.

5

Hint: If one can find a pair of cardinals $\kappa$ and $\lambda$ such that $\kappa\neq\lambda$ but $2^\kappa=2^\lambda$, then $\mathbb Z_2^{(\kappa)}$ and $\mathbb Z_2^{(\lambda)}$ give a counterexample. Since ZFC theory doesn't violate the existence of such pair, I think your conjecture is not true, but I'm not sure if it's false... By the way, this ...

5

If $m = n$, then $0 \to \mathbb{Z}/n\mathbb{Z} \xrightarrow{\operatorname{id}} \mathbb{Z}/n\mathbb{Z} \to 0$ is a free resolution. If $m < n$, it will be useful to write $n = km$, where $k > 1$. The first step of the resolution looks like $F_0 \xrightarrow{\epsilon} \mathbb{Z}/m\mathbb{Z} \to 0$ for some free $\mathbb{Z}/n\mathbb{Z}$-module $F_0$. ...

5

It is enough to prove it preserves short exact sequences: $\;0\to M\to N\to P\to 0$. As the tensor product is right-exact, and $S^{-1}M\simeq M\otimes_A S^{-1}A$, it is even enough to prove it preserves injectivity. So consider an injective morphism $\varphi\colon M\to N$ and suppose $\;S^{-1}\varphi\Bigl(\dfrac ms\Bigr)=0$ in $S^{-1}N$. This means there ...

5

Be careful For a noncommutative ring like this you have to specify what side you are working on (left or right modules.) The answer will hinge on this. Q+A Why can't I use the nilpotent property? The nilpotency of a submodule of $R$ is not useful because module isomorphisms aren't multiplicative, and so the property isn't relevant for modules. ...

5

Proposition: $\mathbb{Q}$ is flat as a $\mathbb{Z}$-module. (Which is to say, the functor $\_ \otimes_{\mathbb{Z}} \mathbb{Q}$ is exact.) Pf: The easiest is to observe that tensoring with $\mathbb Q$ is the same as localizing at $\mathbb{Z} \setminus \{0\}$, and localization is exact. Fancier proof: $\mathbb{Q}$ is the filtered colimit of the free modules ...

5

(I'm implicitly assuming that $R$ is a commutative ring.) An $R[x_1,\ldots,x_n]$-module is the same thing as an $R$-module equipped with $n$ mutually commuting endomorphisms. One way of putting it is that a left $S$-module $M$, where $S$ is a (not necessarily commutative) $R$-algebra, is the same thing as an $R$-module $M$ together with an $R$-algebra map ...

5

Not necessarily. If there is a nontrivial ring honomorphism $\psi: A\to A$, then composing an evaluation homomorphism with $\psi$ gives you something that is not an $A$-algebra homomorphism. For example, in the case $A=\mathbb C$, $n=1$ and $\psi$ being complex conjugation we could have  \phi(a_0 + a_1X + a_2X^2 + \cdots + a_kX^k) \mapsto \overline{a_0} ...

5

If $A$ is a finitely generated abelian group, every subgroup of $A$ is finitely generated (because $A$ is abelian, see proof here). So $A$ is Noetherian as a $\Bbb Z$-module. For Noetherian modules, every epimorphism is an isomorphism, see proof here. We're using that a module is Noetherian if and only if every submodule is finitely generated. The analogous ...

5

A finitely generated $\mathbb Q$-module is a finite-dimensional $\mathbb Q$-vector space. There are elements in $\overline{\mathbb Q}$ of arbitrarily high degree and so $\overline{\mathbb Q}$ cannot be a finite-dimensional $\mathbb Q$-vector space. For instance, $x^n-2$ is irreducible over $\mathbb Q$ for every $n$ and so $2^{1/n}$ is an element of ...

5

The eigenvalues of a matrix $A$ over a field $k$ are the roots of the characteristic polynomial $\chi(A)=\det(\lambda I-A)$. But what is this thing we're taking the determinant of? $\lambda$ is not an element of $k$, but an indeterminate, so to properly describe $\lambda I-A$ we ought to say its coefficients, e.g. $2-\lambda$, are elements of the polynomial ...

5

I believe you are confusing finitely generated modules with finitely generated algebras. In a module we only have addition and scalar multiplication by the base ring, which in this case is $\mathbb Z$. $\frac1 4$ is not in the submodule generated by $1$ and $\frac 1 2$ because $\frac 1 4$ is not an integer linear combination of these. To see that the module ...

5

It does. If $C$ is any chain complex in your additive category and $F = 0$ the zero functor, then $H_n(FC) \cong F\bigl(H_n(C)\bigr)$ for each $n \in \mathbf Z$, as both sides are zero.

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