Questions about commutative rings, their ideals, and their modules.

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What is motivation of combinatorial and commutative algebra? [on hold]

I am so sorry if you feel this kind of question is not appropriate for B.Sc. But I hope you can sympathize with me. Questions: What is different between combinatorics and graph theory? What is the ...
3
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15 views

Cohomological dimension of an arbitrary module.

In the paper, [P, Schenzel, On formal local cohomology and connectedness, J of Alg, 315 (2007), 894--923], he proves the following statement. (Corollary 2.2) Let $M$ be a finitely generated ...
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21 views

What do we call collections of subsets of a monoid that satisfy these axioms?

Consider a monoid $M$ and a semiring $S$. Then there's an $S$-algebra freely generated by the monoid $M$, which can be described explicitly as the set of all finitely supported functions $S \leftarrow ...
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1answer
32 views

An incorrect(?) proof of the Hilbert's Basis Theorem

This is my proof of the Hilber's Basis Theorem. I think it is incorrect. Because it is easier than other proofs. But I can't find out the mistake in my proof. Can anyone help me? Thanks! Claim If ...
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36 views

generalized affine scheme

I'm thinking about following theorem. For a finitaly algebraic theory $\mathbb{T}$, $\text{FP}\mathbb{T}$ denotes the full subcategory of $\mathbb{T}\text{-Alg(Set)}$ consisting of finitely presented ...
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26 views

Kaplansky characterization of principal Artin ring

I would like to learn the proof of this paper of Kaplansky where it is proven that for a commutative ring every module split as sum of cyclic module iff the ring is an Artin principal ideal ring (well ...
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25 views

Which of the algebra isomorphisms hold?

Fix $m, n \ge 1$. Which of the algebra isomorphisms below hold? $k\langle t_1, \dots, t_m\rangle \otimes_k k\langle s_1, \dots, s_n\rangle \cong k\langle t_1, \dots, t_m, s_1, \dots, s_n\rangle$ $k[ ...
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1answer
18 views

Meaning of 3-disjoint

Definition: Two edges $\{x, y\}$ and $\{w, z\}$ of $G$ are said to be 3-disjoint if the induced subgraph of $G$ on $\{x, y, w, z\}$ consists of exactly two disjoint edges. (See page 5 of this file.) ...
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1answer
27 views

Converse of “localization at a prime is local”

Suppose $S^{-1}R$ is the localization of a ring R at a multiplicative subset S, and is local. Must S be the complement of a prime ideal?
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1answer
61 views

Every commutative ring is a quotient of a normal ring?

In the book Étale cohomology by Milne I found on p. 37 (in the context of constructing the henselization of a local ring) the following claim: "Every ring is a quotient of a normal ring". The same is ...
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The completion of the ring of Laurent polynomials with respect to the augmentation ideal.

Let $A = \langle a\rangle$ be an infinite cyclic group on one generator. I'm trying to understand the completion $\widehat{\mathbb{Q}}A$ of the group algebra $\mathbb{Q}A$ with respect to the ...
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1answer
49 views

Embedding tensor product of integral domains

Let $C$ be a subring of integral domains $A,B$ and let $C',A',B'$ denote their field of fractions respectively. Can we always embed $A\otimes_CB$ in $A'\otimes_{C'}B'$ by $a\otimes b\mapsto ...
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37 views

Primitive vectors in $A^n$

Let $A$ be a commutative ring with 1. Let n be a positive integer. I call a vector $(a_1,...,a_n) \in A^n$ primitive, if the ideal generated by $\{a_1,....,a_n\}$ is $A$. Question: Given a primitive ...
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55 views

Axiomatization of the equational theory of ideals in a commutative ring

Is there a known axiomatization of the equational theory of ideal operations in a commutative ring? I have in mind the following: Consider a language with operations for ideal intersection, product, ...
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68 views

Can an element in a Noetherian domain have arbitrarily long factorizations?

I tried to answer this question two days ago. Unfortunately, I said ring, rather than domain, which is what I wanted. So I try again. Let $R$ be a Noetherian commutative domain and let $r\in R$. ...
2
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1answer
27 views

Vanishing of Ext group and Krull dimension

Suppose $A=k[x_1,..,x_n]_{(x_1,..,x_n)}$, it is a regular local ring of dimension $n$. Let $B=A/I$ be a quotient ring of Krull dimension $r$. How to show $\operatorname{Ext}_A^i(B,A)=0$ for ...
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Extending McCoy's theorem to multiple indeterminates [duplicate]

So, working in a commutative ring with unity $R$, I've proven that $f\in R[x]$ is a zero divisor iff there exists $s\in R$ such that $sf=0$. I'm now being asked the followup question to extend ...
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1answer
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Extending valuations and linear disjointness of fields

Let $F$ be a field and let $K$ be a field extension of $F$. Suppose that $K$ can be written as the compositum of field extensions $E$ and $L$ of $F$, linearily disjoint over $F$, thus $K$ can be ...
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Local Constancy of Rank Function

Recently I asked this question. I believe that I have come up with a solution, but I am unsure, because the proof I have seems too easy to be true, and doesn't make very many assumptions. My ...
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1answer
124 views

Can an element in a Noetherian ring have arbitrarily long factorizations?

Suppose $R$ is a Noetherian ring. Is it possible that an element $r\in R$ have arbitrarily long factorizations? That is, for all $n>0$, is there a factorization $r=a_{1n}a_{2n}\cdots a_{nn}$ such ...
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1answer
38 views

Discrete convolution of two sequences

Let $R$ be a commutative ring with unity. A finite sequence $x=\left< x_0,\dots,x_n\right>$ with elements in $R$ is called to be prime if there exists $a_0,\dots,a_n \in R$ such that ...
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Can the torsion of a formal group have rank going to infinity?

Suppose $F(x,y) \in \mathbb{Z}_p[[x,y]]$ is a formal group over $\mathbb{Z}_p$. I denote with $G(-)$ the corresponding functor, so that $G(K)$ will denote for me the maximal ideal of $O_K$ equipped ...
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85 views

Are these quotient modules isomorphic?

Let $K$ be an algebraic number field and $\mathcal{O}_K$ its ring of integers. For a non-zero ideal $\mathfrak{a}$ of $\mathcal{O}_K$ and an element $c \in \mathcal{O}_K \setminus \{0\}$ I wonder ...
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1answer
64 views

Maximal ideals and the projective Nullstellensatz

This is a simple question, but it's one of those things that I've been thinking about so much that I've just kind of lost where I am and need some explicit reference. One of the main corollaries of ...
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Question about points of a variety lying in an extension as K-morphisms

I hope that someone can shed some light on this for me.. or at least point me to some references. Suppose that $X$ is an algebraic (let's just say affine) variety defined over $k$. Suppose I have a ...
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1answer
54 views

Show that $1\otimes (1,1,\ldots)\neq 0$ in $\mathbb{Q} \otimes_{\mathbb{Z}} \prod_{n=2}^{\infty} (\mathbb{Z}/n \mathbb{Z})$.

Show that $1\otimes (1,1,\ldots)\neq 0$ in $\mathbb{Q} \otimes_{\mathbb{Z}} \prod_{n=2}^{\infty} (\mathbb{Z}/n \mathbb{Z})$. Here's what I tried: If $1\otimes (1,1,\ldots)= 0$, then $1\otimes ...
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2answers
62 views

What explicitly is the “adjunction” isomorphism $Hom_C(A,C)\to Hom_B(A,Hom_C(B,C))$?

Suppose $B$ and $C$ are commutative rings, $A$ a $B$-algebra, and $B$ is a $C$-module. What exactly is the "adjunction" isomorphism $$Hom_C(A,C)\to Hom_B(A,Hom_C(B,C))?$$ Given $A\to C$, it needs to ...
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33 views

Maximal ideal of polynomial ring over a subfield

Let $L/K$ be an algebraic extension of fields. Let $B = L[X,Y]$ and $A = K[X,Y]$. Suppose $a$, $b \in L$ and $m = (X-a,Y-b)$ is an ideal of $B$. Show that $m$ and $m \cap A$ are maximal ideals of ...
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1answer
28 views

Gauss lemma for arbitrary commutative ring [duplicate]

Part (iv) of exercise #2 for chapter 1 in Atiyah and Macdonald's book Introduction to Commutative Algebra asserts that if $f, g \in A[x]$ are primitive then $fg$ is primitive. We know that this is ...
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1answer
52 views

Does $f\otimes_A 1_{A/m}:M\otimes A/m\to N\otimes A/m$ injective for all maximal $m$ imply $f$ is an isomorphism?

Let $A$ be a commutative ring. Suppose $f\colon M\to N$ is a morphism of free $A$-modules of equal, finite rank. If $f\otimes_A 1_{A/m}$ is injective for all maximal ideals of $A$, does this imply ...
3
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1answer
24 views

Let $A$ be a finitely generated $R$-module. Show that if $A/MA$ can be generated by $n$ elements then so can $A$.

Let $R$ be a local commutative ring with the maximal ideal $M$. Let $A$ be a finitely generated $R$-module. Show that if $A/MA$ can be generated by $n$ elements then so can $A$. I tried to apply ...
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1answer
23 views

Finite ring extension of local rings

Let $R$ and $S$ be local rings with the maximal ideals $M$ and $N$, respectively. Assume that $R\subset S$ and that $S$ is a finitely generated $R$-module. If there exists a proper ideal $I$ of $R$ ...
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MCM Modules over Cyclic Quotient Singularities

Let $k$ be a field and $R$ the ring $k[[u^{n+1}, uv, v^{n+1}]]$. Then the indecomposable MCM $R$-modules are given by $M_j = R(u^av^b \vert b-a\equiv j \mod{n+1})$ for $j = 1,\ldots, n$. This is of ...
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How does commutative and/or differential algebra think about total derivatives?

If we apply the "operator" $\frac{d}{dx}$ to the polynomial $xy$, we get the expression $y+x\frac{dy}{dx}.$ (Source: high school.) Thinking of $xy$ as an element of the polynomial ring ...
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1answer
72 views

Dimension of quotient ring

What is the dimension of the following quotient ring, $\mathbb{Z}[x,y,z]/\langle xy+2, z+4 \rangle$, where $\mathbb{Z}$ is the ring of integers? I realized this is isomorphic to ...
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1answer
28 views

Prove that if $M$ is a simple $R=k[x_1,…,x_m]$ -module, then the dimension of $M$ over $k$ is finite.

Let $k$ be a field and let $R=k[x_1,...,x_m]$ be the polynomial ring in $m$ indeterminates. Prove that if $M$ is a simple $R$-module, then the dimension of $M$ over $k$ is finite. I think since ...
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39 views

characterization of integral closure?

I would like to know whether there exists any characterization of integrally closed domains which is related to some morphisms construction with $\mathbb{Z}$ and $\mathbb{Q}$. I was thinking about ...
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Deciding whether a non-f.g. non-divisible flat module is projective or not.

Assume $S= R[T]/(f)= R[w]$ is a flat non-divisible $R$-module, where $R$ is a noetherian UFD, $T$ is an indeterminate over $R$, and $f\in R[T]$ is a non-monic polynomial of positive degree. Can we ...
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1answer
54 views

The direct limit of morphisms and the direct limit of tensor product functors

While reading these notes I had something of an existential crisis, after realizing that my understanding of direct limits might somehow be fundamentally insufficient. In particular, alarms started ...
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Every ideal has a FFR

Let $A$ be a regular local ring. Then every ideal has a finite free resolution. My thoughts: it's easy to prove that every ideal $I$ has a free resolution. In fact $I$ is finite and there is a ...
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1answer
25 views

Reduced one-dimensional Noetherian ring is Cohen-Macaulay

If $(R,m)$ is a local Noetherian reduced ring of Krull dimension $1$ then $R$ is Cohen-Macaulay, since in a reduced Noetherian ring the set of zero divisors is the (finite) union $U$ of minimal prime ...
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1answer
131 views

Algebraic extension and Krull dimension

Let $A \subseteq B$ be an extension where $A,B$ are Noetherian, commutative rings. If $B$ is algebraic over $A$, can we say that $\dim B\leq\dim A$? Just read the following paper "Constructive ...
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62 views

Jacobson radical of $\mathbb{F}_{2}(t)[x]/(x^4-t^2)$

Let $\mathbb{F}_{2}$ be the field of two elements. Let $R=\mathbb{F}_{2}(t)[x]/(x^4-t^2)$. Why is $R/J(R)$ equal to $\mathbb{F}_{2}(t)[x]/(t-x^2)$? here $J(R)$ denotes the Jacobson radical of $R$.
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Jacobson radical of an indecomposable commutative ring

Let $R$ be a commutative indecomposable ring with identity which has infinitely many maximal ideals. Can we deduce that the Jacobson radical of $R$ (the intersection of all maximal ideals) is the zero ...
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What is the obstruction to extending a linear map on tangent spaces of a variety to a regular map on neighborhood?

Suppose that $X$ and $Y$ are algebraic varieties of the same dimension $n$. If $p$ and $q$ are points in $X$ and $Y$ respectively, suppose that there is a linear map $i : T_p X \to T_q Y$. My vague ...
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1answer
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The last nonzero local cohomology module is not finitely generated. [closed]

Let $R$ be a Noetherian ring and $I$ an ideal of $R$. If $M$ is a finitely generated $R$-module and $i\neq 0$ is the greatest integer such that $H^i_I(M)$ is nonzero, then $H^i_I(M)$ is not a ...
3
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1answer
61 views

What's the kernel of the codiagonal $k[x] \otimes_k k[x] \rightarrow k[x]$?

maybe this question is really stupid, but I could not solve it after thinking for a while. Let $I$ be the kernel of the codiagonal map $$k[x] \otimes_k k[x] \rightarrow k[x]$$ given by $f(x) \otimes ...
3
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1answer
39 views

Fraction field of $F[X,Y](f)$ isomorphic to $F(X)[Y]/(f)$

Assume $F$ is a field and $f$ is an irreducible polynomial in $F[X,Y]$ which involves the variable $Y$. Then, by Gauss's lemma, $f$ is irreducible also in $F(X)[Y]$ so that $F(X)[Y]/(f)$ is a field ...
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1answer
24 views

A Cohen-Macaulay localisation

Let $R=\mathbb C[X,Y]/(Y^3-X^3)$, let $x,y$ be the images of $X,Y$ in $R$, and let $R_1$ be the localization of $R$ at the maximal ideal $(x,y)$. I want to prove that $R_1$ is a Cohen-Macaulay ...
3
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48 views

Determining prime ideals lying above a given ideal

Let $R=\mathbb{Z}[x]/(f)$, where $$f(x)=x^4+42x^3-11x^2+22x-2002002002002002.$$ Let $I=3R$, the ideal generated by $3$ in $R$. Find all prime ideals of $R$ that contain $I$. I am hoping to ...