Questions tagged [ac.commutative-algebra]
Commutative rings, modules, ideals, homological algebra, computational aspects, invariant theory, connections to algebraic geometry and combinatorics.
5,495 questions
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When does the group of invertible ideal quotients = the free abelian group on the prime ideals?
I haven't learned that much about primary decomposition, but from I understand about Dedekind domains, we have that all fractional ideals are invertible and all (plain old) ideals factor uniquely into ...
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1
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Factor $\sum_{n=1}^{N} x^n$ [closed]
I am attempting to factor an $N^{\text{th}}$ degree polynomial with coefficients strictly equal to $1$ given by the equation
$$\sum_{n=1}^{N} x^n$$
Although the Galois group for anything beyond a ...
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21
votes
2
answers
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What properties define open loci in excellent schemes?
Let $R$ be an excellent Noetherian ring. A property $P$ is said to be open if the set $\{q \in \operatorname{Spec}(R) \ | \ R_q \ \text{satisfies} \ (P)\}$ is Zariski open. Examples of open ...
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Source for conjectures in commutative algebra
Do you know some books/survey papers/ websites on conjectures or open problems in commutative ring theory? I want to see if
there are very famous open problems or conjectures in commutative ring ...
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0
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Cohomological dimension and height of ideals
Let $I$ be an ideal in a Noetherian ring $R$. We define the cohomological dimension of $I$ to be $\operatorname{cd}(I)=\operatorname{sup}\{i\in \mathbb N:\operatorname{H}_I^i(R)\neq0\}$ and it is ...
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Can "$\exists\mathcal{X}(R\cong C(\mathcal{X}))$" be expressed in "large" infinitary second-order logic?
Originally asked and bountied at MSE without success:
Say that a ring $R$ is spatial iff there is some topological space $\mathcal{X}$ such that $R\cong C(\mathcal{X})$, where $C(\mathcal{X})$ is the ...
CommunityBot
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86
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Buchsbaum-Eisenbud-Horrocks conjecture for finitely generated modules
Someone know a version for conjecture of Buchsbaum-Eisenbud-Horrocks whithout the assumptions that
the $R$-module $M$ has length finite?
1
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0
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A question about minimal system of generators and regular sequences
Let $(R,\mathfrak{m},k)$ a Noetherian local ring and $I$ an ideal. Suppose that:
$\mu(I)=\operatorname{grade}(I,R)+1$ and $\operatorname{pd}_R(R/I)=\operatorname{grade}(I,R)$. (Some people says $I$ is ...
- 33
5
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1
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Localization of a ring and the Hom functor
Let $R=\mathbb{Z}[x,x^{-1}]$ be the ring of Laurent polynomials in $x$, $\mathfrak{p}=(1-x)$ be an ideal in $R$ and $R_\mathfrak{p}$ be the localization. I want to know what $\text{Hom}_R(R_\mathfrak{...
- 6,262
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Some properties for height 1 prime ideals in the local ring
Let $\mathbb{K}$ be an algebraically closed field of characteristic $0$. Let $R=\mathbb{K}[x_0,x_1,\dotsc,x_n]/I$ be the coordinate ring of an affine variety/projective variety. Also, assume that $I$ ...
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Maps between K-groups induced by rings homomorphism
Let $f: R\to S$ be a map between two commutative Noetherian rings. Let $G_0(R)=K_0(mod R)$ be the Grothendieck group of finite generated modules over $R$. It means $G_0(R)$ is the quotient of the free ...
- 2,821
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1
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On the trivialization of the sheaf of kahler differentials on the G-invariant topology
Let $X$ be a connected, smooth affine algebraic variety over an algebraically closed field $K$ of characteristic zero. Assume we have a finite group $G$ acting on $X$ by morphisms of $K$-schemes. ...
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Are units of rings of functions on algebraic varieties finitely generated (mod. constants)?
Hello,
Consider the following question. Let $A$ be a finitely generated reduced algebra over an algebraically closed field $k$. Consider the group of units of $A$, modulo the group $k^*$. Is this ...
- 14.2k
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1
answer
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Brauer group of rational numbers
Reading about the calculation of the Brauer group of rational numbers, the calculations of the group are extremely lengthy and technical. First of all, it will be very helpful to me if someone can ...
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0
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Characterise set of polynomials which are zero over an ideal
This is not a specific question, but rather a question about possible techniques approaching a problem. Although this question came from research, it might not fit this forum; in which case I will ...
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Let $R$ be a non-catenary, and $f: R \to S$ be a finite monomorphism. Can $S$ be catenary?
Let $R$ be a commutative ring with identity. Then $R$ is $\textit{catenary}$ if for each pair of prime ideal $p \subsetneq q$, all maximal chains of prime ideals $p = p_0 \subsetneq p_1 \subsetneq \...
- 1,355
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5
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When a formal power series is a rational function in disguise
Given a formal power series $f \in k[[X]]$, where $k$ is a commutative field, is there any good way to tell whether or not $f\in k(X)$?
Edit: To clarify, "good way to tell" means "computable ...
- 1,446
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Does a torsion-free coherent sheaf embed into a locally free sheaf?
Let $ X $ be a Noetherian integral regular scheme and $ \mathcal{F} $ be a torsion-free coherent sheaf. (One definition of torsion-free is that the natural map $ \mathcal{F} \rightarrow \mathcal{F} \...
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The index of an order defined by a binary form
In his well-known paper, Nakagawa generalized a construction due to Birch and Merriman to arbitrary binary forms and orders. In particular, his construction gives a canonical algebraic order $\mathcal{...
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2
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1
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Relation between free resolutions and minimal free resolutions
Let $(R,\mathfrak{m},k)$ a Noetherian local ring and $M$ a finitely generated $R$-module. In this case, given $F_{\bullet}$ a free resolution of $M$, what is the relation between $F_{\bullet}$ and a ...
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Faithful module cancellation with maximal ideal
Let $k$ be a field of characteristic $0$ and $R = k[[x_1, \dotsc, x_n]]$. Suppose that $M$ is a faithful, finitely generated $R$-module and $\mathfrak{a} < R$ is an ideal such that $\mathfrak{a} M =...
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How much theory works out for "almost commutative" rings?
I've been reading about D-modules, and have seen a proof that D_X, the ring of differential operators on a variety, is "almost commutative", that is, that its associated graded ring is commutative. ...
- 1,446
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235
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Algebras which admit tensor calculus and (pseudo-)Riemannian geometry
It's an often observed fact that the basic notions of analysis on manifolds and (pseudo-)Riemannian geometry, such as tensors, connections and curvature, can be defined in purely algebraic terms. The ...
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1
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Rational even polynomials maximally tangent to the unit circle
This question is motivated by College Mathematics Journal problem 1196, proposed by Ferenc Beleznay and Daniel Hwang. My solution to this problem (pre-publication version here) uses Chebyshev ...
- 109k
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1
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Is this a true weakening of the quasi-coherence property?
Let $R$ be a commutative Noetherian ring, and $X=$Spec$(R)$ the associated affine scheme. Let $F$ be a sheaf of $O_X$-modules. Consider the following condition
(#) For all containments $V \subseteq ...
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Meaning of "cut out (scheme-theoretically)"
Let $V$ be a projectively normal closed subvariety of some projective space over an algebraically closed field $\mathbb{K}$. Let $R$ be the local ring at the vertex of the affine cone over $V$ ($R$ is ...
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Hypermodulus and what mathematical objects have it
When researching divergent integrals, I decided to introduce a concept of "modulus" or "determinant" of divergent integral (and series). Basically, it is the exponent of the real ...
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What do epimorphisms of (commutative) rings look like?
(Background: In any category, an epimorphism is a morphism $f:X\to Y$ which is "surjective" in the following sense: for any two morphisms $g,h:Y\to Z$, if $g\circ f=h\circ f$, then $g=h$. Roughly, "...
- 5,039
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Strict henselianization of complete intersections
As far as I understand (and tbh for my purposes), one of the main points of strict henselisation of a local ring is that it computes the stalk at a point of a scheme in the étale topology. In the ...
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Theorem on formal functions when the initial data is a proper map of formal schemes
Let $\pi: X \to S:=\mathrm{Spf}\text{ } A$ be a proper morphism of $\mathbb{Z}_p$-admissible formal schemes and $\mathcal{F}$ be a coherent sheaf on $X$.
Set $S_0=\{x\}$ be a closed point of $S$ and $...
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0
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Jacobian ideal as primary idea;
Let $R = \mathbb{C}\{x_1, \dots,x_n\}$ be the ring of germs of analytic functions and let $f \in R$ be a homogeneus polynomial of degree $p$ such that $\sqrt{Jac(f)}=\langle x_1, \dots, x_{n-1}\rangle ...
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Computer algebra programs that can solve polynomial systems on algebraically closed fields besides MAGMA
I was wondering which computer algebra programs out there can solve polynomial systems on the algebraic closure of $\mathbb{Q}$ analytically and efficiently.
So far, I only found MAGMA with its ...
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Bertini type result for torsion-freeness
Let $R$ be a local, regular $\mathbb{C}$-algebra and $\mathfrak{m}$ be the maximal ideal. Let $M$ be a finitely generated torsion-free $R$-module. Suppose there exists $f \in \mathfrak{m}$ such that $...
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0
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question about Sinnott's proof of the Ferrero-Washington Theorem
I'm currently reading the paper "On the $\mu$-invariant of the Γ-transform of a rational function" by W Sinnott. In this paper, he gives an alternate proof that $\mu=0$ for abelian number ...
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Coprime multivariate polynomials
Let ${\bf R}$ be a gcd domain, $n \geq 2$, $k \in \mathbb{N}^*$, and $f,g \in
{\bf R}[X_1,\ldots,X_n]$. Supposing that $f$ and $g$ are coprime in ${\bf R}[X_1,\ldots,X_n]$, that is, $\gcd(f,g)=1$, ...
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Prove that $\overline{a}_{11}$ is a prime element in $R$
Consider the affine space given by four $2\times 2$ matrices, i.e., $\mathbb{A}^{16}\cong M(\mathbb{C})_{2\times 2}^4$. Now, consider the algebraic set $V$ given by the vanishing of the relation $AB-...
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An exercise in fuzzy logics built from a t-norm [closed]
Consider the following t-norm:
$$
a * b = \begin{cases}
2ab, &\quad\text{if }a, b\le1/2\\
\min\{a, b\} &\quad\text{otherwise}
\end{cases}
$$
We build from it the $\...
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Formal power series is Taylor expansion of rational function iff Hankel determinants vanish?
Let $$ u(T)=\sum_{n = 0}^\infty a_nT^n$$ be a formal power series over a field $K$. Then why does $u(T)$ lie in $K(T)$ (i.e. is the Taylor expansion of a rational function) if and only if there is an $...
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Which power series in $\mathbb{Z}_p[[T]]$ are rational functions? [duplicate]
Consider the power series ring $\mathbf{Z}_p[[T]]$, where $\mathbf{Z}_p$ denotes the $p$-adic integers. I'll call a function $f(T) \in \mathbf{Z}_p[[T]]$ a rational function if I can write it as:
$$f(...
- 1,949
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Is the ring of power series with coefficients in a field free as a module over the polynomials subring? [closed]
Is the ring of power series with coefficients in a field free as a module over the polynomials subring?
- 12.9k
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1
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Tensor product of rings of Witt vectors
Let $A$, $B$, and $C$ be commutative rings such that $A\otimes_C B$ makes sense. If $W_n(A\otimes_C B), W_n(A), W_n(C),$ and $W_n(B)$ are the length $n$ Witt vectors of the rings $A,B,C,$ and $A\...
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1
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On presentations of universal rings of deformations
Let $k$ be a finite field of characteristic $p$, and $R$ a complete local noetherian algebra with residue field $k$. It is well known that $R$ has a natural structure of an algebra over the ring Witt ...
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Is $\mathbb{Z}[i,\varphi]$ a Euclidean domain?
I've already asked this question on Math StackExchange but having gotten no responses this may be more obscure than I had initially believed.
Here $\varphi=\frac{1+\sqrt{5}}{2}$. It's true that $\...
- 465
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0
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155
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Homogeneous deformation of isolated singularities
Let $f\in \mathbb{C}[x_1, \dots,x_n]$ be a homogeneous polynomial of degree $p$ and let $F \in \mathbb{C}[x_1, \dots,x_n,t]$ be a polynomial such that $F(x_1, \dots, x_n,0)=f$, for every $t_0$ we have ...
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4
answers
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Generators of a maximal ideal of $k[X_1,\cdots,X_n]$
Given $k$ an algebraically closed field, I know that that a maximal ideal $\mathfrak{m}$ of $A = k[X_1,\cdots,X_n]$ is just a $\langle X_1-a_1,\cdots,X_n-a_n \rangle $ (Nullstellensatz).
Knowing that, ...
- 4,713
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1
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331
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Example of non vanishing Ext
Let $R$ be a commutative Noetherian ring and $I$ is a proper ideal of $R$. suppose that $M$ is a f.g. $R$-module.
$\DeclareMathOperator\Ext{Ext}$I'm looking for an example that has this property:
$$\...
- 533
1
vote
1
answer
174
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Completion reducing to localization on Noetherian rings
It is quite easy to show that if $A$ is a Dedekind domain and $\mathfrak{p}\in \operatorname{Spec} A$, then if $A_{\mathfrak{p}}$ is the completion of $A$ at $\mathfrak{p}$ and $A_{(\mathfrak{p})}=(A\...
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In $\mathbb{Z}[G]$, $G\cong \mathbb{Z}^r$, does $f\cdot g\geq 0$ imply $f\geq0$?
Let $G=\mathbb{Z}^r$ be a free abelian group, and $\mathbb{Z}[G]$ be the group ring of $G$. Define a partial ordering $\leq$ on $\mathbb{Z}[G]$ by
$$\sum_{g\in G}n_g[g]\leq\sum_{g\in G}n'_g[g]\iff n_g\...
- 2,902
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0
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123
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Clarification about theorem on vanishing polynomials
The theorem below is from page 3 in the this paper on polynomials in $\mathbb{Z}_m[x]$.
Let $F$ be a polynomial in $\mathbb{Z}_m[X]$. Then $f \equiv 0$ iff $$F \equiv F_nS_n + \sum_{k=0}^{n-1}a_k(m/(...
- 1,561
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votes
2
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What is the insight of Quillen's proof that all projective modules over a polynomial ring are free?
One of the more misleadingly difficult theorems in mathematics is that all finitely generated projective modules over a polynomial ring are free. It involves some of the most basic notions in ...
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