All Questions
6,055 questions
2
votes
1
answer
91
views
Is the integral closure of a $\mathbb{Z}/n\mathbb{Z}$-graded noetherian domain in a bigger $\mathbb{Z}/n\mathbb{Z}$-graded domain also graded?
Let $A\subset B$ be an inclusion of $\mathbb{Z}/n\mathbb{Z}$-graded noetherian domains. Is the integral closure of $A$ in $B$ also $\mathbb{Z}/n\mathbb{Z}$-graded?
This is true for the $G$-graded case ...
- 16.2k
0
votes
0
answers
226
views
Geometric interpretation of normalization inside a finite extension of function field
$\DeclareMathOperator\Spec{Spec}$Suppose $X = \Spec A$ is a smooth affine variety over $\mathbb C$ and suppose $L/K$ is a finite extension of its function field. Let $Y = \Spec B$, where $B$ is the ...
- 12.9k
7
votes
0
answers
295
views
A minimal semigroup generating subset of the additive reals
I asked this on MSE, but I was told to ask it here because it is a difficult question. Consider the additive magma of the real numbers, $(\mathbb{R};+)$. Does there exist a subset $S$ of the reals ...
- 63.9k
4
votes
1
answer
141
views
Kernels of actions on truncated polynomial algebra
Let $p$ be an odd prime, and let $k=\mathbb{F}_p$ be the field with $p$ elements. Let $G=\text{GL}_n(k)$. The group $G$ acts on the truncated polynomial algebra $A:=k[x_1,\ldots, x_n]/(x_1^p,\ldots, ...
- 16.2k
3
votes
0
answers
264
views
The points of $\operatorname{Spa}\mathbb{Z}_p$
$\DeclareMathOperator\Spa{Spa}$What are the points of $\Spa\mathbb{Z}_p$? I read in Scholze-Weinstein that this adic spectrum consists of 2 points, a special point, which corresponds to the pullback ...
- 63.9k
5
votes
0
answers
160
views
Cohen-Macaulayness of rings of polynomials vanishing at points
Let $V$ be a finite dimensional vector space, let $L_1$, $L_2$, ..., $L_r$ be subspaces and let $w_1$, $w_2$, ..., $w_r$ be positive rational numbers. Define a graded ring $R$ where $R_d$ is those ...
- 156k
3
votes
0
answers
70
views
Degree of an even/odd part of a formal power series over a polynomial ring
Let $K$ be a field with $\operatorname{char}K\ne 2$ (say, $K=\mathbb{R}$ or $\mathbb{C}$) and consider a formal power series $f=f(x)\in K[[x]]$ such that $[K[x,f]:K[x]\,]=d$. Suppose $f_e,f_o\in K[[x]]...
- 1,190
4
votes
2
answers
705
views
Why are canonical modules supported everywhere?
Let $A$ be a local CM ring, and $\omega$ a canonical module of $A$. Here are two properties of $\omega$ from Bruns & Herzog:
$\omega_{\mathfrak{p}}$ is a canonical module of $A_{\mathfrak{p}}$ ...
- 2,821
3
votes
0
answers
280
views
The closed unit adic disk
I am reading the Scholze-Weinstein Berkeley lecture notes on "Perfectoid Spaces", and in particular I am stuck trying to understand the closed adic unit disk, which is the second example of ...
- 2,907
2
votes
0
answers
265
views
What is the residue field of the integer ring of $\mathbb{C}_p$?
Fix a prime $p$. Let $\mathbb{C}_p$ be the completion of the algebraic closure of $\mathbb{Q}_p$, and $\mathcal{O}_{\mathbb{C}_p}$ the integer ring of $\mathbb{C}_p$. I know $\mathcal{O}_{\mathbb{C}_p}...
- 63.9k
-3
votes
1
answer
208
views
can the square of a polynomial be written as a sum of squares of polynomials with only even degree terms?
Let $f$ be a polynomial with real coefficients in several indeterminates $x_1, \dots, x_n$. Suppose that
$$ f = g^2 $$
for some polynomial $g$.
Is it true that we can find polynomials $h_1, \dots, h_m$...
- 785
0
votes
0
answers
53
views
When a given set of primes of height 1 is a set associated primes of an element
Let $R$ be a Noetherian local ring of dimension $\geq 3$ and $\{p_1,\ldots , p_n\}$ be a collection of prime ideals of height $1$. Does there exist an element $x\in R$ such that $Ass(R/xR)=\{p_1,\...
- 1,713
1
vote
0
answers
48
views
Neighborhoods of idempotents in topological inverse semigroups
In a topological group, for any neighborhood $U$ of the origin, there is another such neighborhood with the property that $V.V\subseteq U.$ I conjecture a similar property for topological inverse ...
- 63.9k
1
vote
0
answers
109
views
On Serre's condition and singular locus of determinantal rings
Let $R$ be a Commutative Noetherian ring. Let $\mathbf X:=[X_{ij}]_{1\le i \le r, 1 \le s \le t}$ be a matrix of indeterminates. Let $t>1$ be an integer, and $I_t(\mathbf X)$ denote the ideal in $...
- 413
4
votes
2
answers
295
views
Local Ext for reflexive sheaves on surfaces
Let $X$ be a normal Gorenstein complex surface with $H^i(X,\mathcal{O}_X)=0$ for $i>0$ and $F$ be a rank one reflexive sheaf on $X$. I'm trying to find some ways to determine local Ext $\mathcal{E}...
- 20.5k
6
votes
0
answers
292
views
What is the algebra structure on the pushforward of the structure sheaf along a finite map to $\mathbb{P}^1$?
$\newcommand{\P}{\mathbb{P}}\newcommand{\O}{\mathcal{O}}$ Let $f : C \to \P^1$ be a ramified finite map of degree $d$ of smooth algebraic curves over an algebraically closed field $k$. How can we ...
- 605
1
vote
0
answers
79
views
What is $\text{Hom}(\mathcal{F}\times\mathcal{G}, \mathbb{A}^1)$?
Let $S$ be an affine scheme and let $\text{Aff}(S)$ be the site of affine $S$-schemes. Let $\mathcal{F}$ and $\mathcal{G}$ be a pair of sheaves on $\text{Aff}(S)$, and let $\mathbb{A}^1_S$ be the ...
- 2,907
3
votes
1
answer
173
views
$\Omega$ for noetherian semiperfect rings
Let $A$ be a a two-sided noetherian semiperfect ring and assume that the injective dimension of the left and right regular modules are equal to $n \geq 1$.
Let $\Omega^n(mod A)$ be the category of $n$-...
- 16.2k
3
votes
0
answers
119
views
Finite algebras with finitely many automorphisms
Let $B'/B$ be a finite locally free algebra. Locally in $B$, there is an isomorphism of $B$-modules $B'\simeq B^{\oplus n}$. When is the automorphism group of $B'/B$ finite? When is it unramified? Is ...
- 1,094
37
votes
1
answer
1k
views
If $A$, $B$ are abelian groups such that $\mathrm{Hom}(A, G) \cong \mathrm{Hom}(B, G)$ for all abelian groups $G$, must $A$ and $B$ be isomorphic?
$\DeclareMathOperator\Hom{Hom}$The question is in the title. If the isomorphism $\Hom(A, G) \cong \Hom(B, G)$ is natural in $G$ then this is just the Yoneda Lemma. If $A$ and $B$ are finitely ...
- 12.9k
1
vote
0
answers
136
views
On Noetherianity and local ness of a completed tensor product
Let $R$ be a regular local complete (with respect to the maximal ideal) ring with field of fraction $K$. Let $S\cong R[[x_1,\cdots, x_n]]/J$ (this is a Noetherian local ring which is an $R$-algebra) ...
- 413
1
vote
0
answers
180
views
Questions about the article "A tour of the strong and weak Lefschetz properties"
I'm trying to learn a little about the Lefschetz properties and to start off have been reading Migliore and Nagel's survey article: https://arxiv.org/abs/1109.5718. I'm new to the area, and I have a ...
- 11
44
votes
4
answers
3k
views
Do rings of smooth functions differ from rings of continuous functions?
Let $M$, $N$ be connected nondiscrete compact smooth manifolds. Can the ring of continuous functions on $M$ be isomorphic to the ring of smooth functions on $N$?
CommunityBot
- 1
5
votes
1
answer
359
views
Constructive proof of univariate McCoy theorem without Dedekind-Mertens?
McCoy's theorem (one of them) says that for any commutative ring $A$, $f\in A[x]$ is a zero-divisor iff it's annihilated by a scalar in $A$.
There's a widespread proof by contradiction. There's also a ...
- 33.8k
4
votes
1
answer
277
views
Is there a good notion of kernels of quadratic forms on abelian groups?
Let $G$ be an abelian group and let $q:G \to \mathbb{Q/Z}$ be a quadratic form, i.e. $q(a)=q(-a)$ and $b(x,y)=q(x+y)-q(x)-q(y)$ is a bihomomorphism. On vector spaces, when people speak about the ...
- 2,928
0
votes
1
answer
221
views
Are zero dimensional ideals radical?
I have a question about Theorem 3.7.25. of Computational commutative algebra I by M. Kreuzer and L. Robbiano.
Let $K$ be a perfect field, $I \subseteq K[x_1, \ldots, x_n]$, be a zero dimensional ...
- 121
3
votes
0
answers
112
views
Local cohomology with coefficients in ideals of parameters
I'm not an expert in local cohomology, but the following problems have come up in my work, and I'd like to get a sense of where things stand.
Let $\mathbb{A}^n=\operatorname{Spec} \mathbb{C}[x_1, \...
- 63.9k
3
votes
0
answers
161
views
Making the powerset into a topological monoid
Every monoid $X$ induces a monoid structure $\circledast$ on $\mathcal{P}(X)$ via
$$U\circledast V := \{uv\ |\ u\in U,v\in V\}.$$
Moreover, a morphism of monoids $f\colon X\to Y$ induces a morphism of ...
- 1,074
0
votes
0
answers
172
views
When does this commutative non-associative algebra have nilpotent elements?
Consider a non-associative commutative unital algebra of finite dimension where the product is defined by a Cayley table such that elements are generated with real number coefficients
$(a_0, \dotsc, ...
- 763
1
vote
0
answers
162
views
Difficulty understanding a step in the proof of multiset version of Cauchy-Davenport Theorem
In a paper "G. Kós, L. Rónyai, Alon’s Nullstellensatz for multisets, Combinatorica, 32(5) (2012) 589-605", the authors prove a multiset version of the Cauchy-Davenport Theorem (please see ...
- 167
3
votes
3
answers
1k
views
Formal power series vs localization at non-constant polynomials
Let $A$ be a commutative ring.
On one hand we have the completion $ A[\![ x ]\!]$, given by the ring of formal power series. Elements are of the form $\sum_k a_kx^k$. The Jacobson radical of $ A[\
- 44.7k
1
vote
0
answers
95
views
Eventual stabilization for repeatedly adding multiplayer games
This question is an outgrowth of a couple previous questions of mine. In order: 1,2,3. This should be fully self-contained, but those questions may help motivate this one.
To keep things readable, I'...
- 20.5k
3
votes
1
answer
284
views
Is this functor $\mathcal{F}: \text{Sch}/\mathbb{Q}\longrightarrow \text{Sets}$ a sheaf?
Consider the functor $\mathcal{F}: \text{Sch}/\mathbb{Q}\longrightarrow \text{Sets}$, defined by sending a scheme $X$ with coordinate ring $\mathcal{O}(X)$ to the set of orbits $B(\mathcal{O}(X))\...
- 156k
1
vote
0
answers
159
views
Cohomology of modular curve
(A follow-up on this). Consider the modular curve $X_0(N)$. I'm trying to make the jump from understanding the cohomology $H^1(X_0(N), \mathbb{Z})$ to understanding $H^1(X_0(N), \mathcal{O})_\mathfrak{...
- 223
12
votes
2
answers
785
views
Is the Petersen graph a "Cayley graph" of some more general group-like structure?
The Petersen graph is the smallest vertex-transitive graph which is not a Cayley graph. Is it the "Cayley graph" of some slightly more general group-like structure?
- 1,926
2
votes
0
answers
232
views
Context for Wiles defect criterion and patching
This is not a homework or a project question, just me trying to get acquainted to the subject: I am an MSc student who recently came across the Wiles defect numerical criterion (see, for example, ...
- 223
6
votes
1
answer
288
views
What is the right level of generality for $(R/a) \times (R/b) \cong (R/\gcd(a,b)) \times (R/\operatorname{lcm}(a,b))$?
Let $R$ be a principal ideal domain. Let $a$ and $b$ be two elements of $R$. Let $g$ be a greatest common divisor of $a$ and $b$, and let $\ell$ be a least common multiple of $a$ and $b$. (Of course, ...
- 28.7k
2
votes
0
answers
108
views
Deformation to normal cone of the exception divisor of a log-resolution
I am reading the paper Iterated vanishing cycles, convolution, and a motivic analogue of a conjecture of Steenbrink due to G. Guibert, F. Loeser, and M. Merle. The main tool, like a lot of papers in ...
- 883
2
votes
1
answer
640
views
How to show that the intersection of two certain affine varieties is reduced?
$\DeclareMathOperator\codim{codim}$Let $X=V(I)$, $Y=V(J)$ be two affine varieties. I'd like to know possibile strategies to understand when their schematic intersection, i.e. $X\cap Y=V(I+J)$, is ...
- 12.9k
1
vote
0
answers
116
views
Inclusion between rings after localization
Let $\phi:A \to B $ an injective finite ring map between two noetherian integral domains $A,B$. Let $ C \subset B$ a subring of $B$ and assume that there exist a prime ideal $\mathfrak{p} \subset A$ , ...
- 6,038
4
votes
1
answer
947
views
Tensor product and homomorphism
Let $A$ be a commutative ring, and $M$ be an $A$-module, and $M^*$ be $\mathrm{Hom}_A(M,A)$. Let $f$ be the map from $M \otimes_A M^*$ to $\mathrm{Hom}_A(M,M)$, such that, for all $x=\sum_i a_i \...
- 63.1k
8
votes
2
answers
2k
views
Is being reduced a generic property of schemes?
(Naive formulation:) Let $X$ be an (irreducible) affine variety (over an algebraically closed field $k$) and $I$ be an ideal of the coordinate ring $R$ of $X$. Assume $Y = V(I)$ is equidimensional. ...
- 42.9k
5
votes
3
answers
508
views
Absolute integral closure of local UFD
Let $R$ be a Nagata Noetherian local UFD, and $K$ be its fraction field. I wonder if its absolute integral closure $R^+$, which is the integral closure of $R$ in $K^\text{sep}$, is flat over $R$. Let $...
5
votes
1
answer
251
views
Monoid associated to $>2$-player Hackenbush
There is some literature on multiplayer combinatorial game theory, but as far as I can tell none of it follows the line of attack below. I'd love a pointer to a similar approach taken in the ...
- 20.5k
1
vote
1
answer
120
views
How to compute the associated reduced ring for this finitely generated algebra?
Let $k$ be a field, $m$ be a positive integer and $R$ be the subring $k[x,xy,xy^2,…,xy^m]$ of the polynomial ring $k[x,y]$. Let $B$ be the quotient ring $R/xR$. Then $B$ is the finitely generated $k$-...
- 6,262
1
vote
0
answers
132
views
A question concerning cancellation of ideals
I am working on a number theory project, and at one stage, I encounter a commutative algebra problem. Vaguely speaking, my hope is to show that two ideals are equal. Now I shall explain the data I am ...
- 11
1
vote
2
answers
301
views
Completions of non-Noetherian rings with respect to finitely generated ideal
All the standard counterexamples to flatness that I have seen involve completions with respect to non-finitely generated ideals.
I am interested in the following two cases:
Let $A$ be a local ring ...
- 38k
1
vote
1
answer
133
views
Whether equality of two sections of an integral affine scheme can be check fiberwise?
If $X\rightarrow S$ is a morphism between two integral affine schemes, and $a,b\in X(S)$ are two sections. Assume that $a$ and $b$ agree after base change to each geometric point of $S$, are they the ...
- 2,665
2
votes
1
answer
288
views
When a sum of the ideals is radical
Let $X,Y$ be algebraic subsets of $\mathbb A^n.$ I would like to show that if $X$ and $Y$ intersect "transversely" then $I(X)+I(Y)$ is radical (so $I(X\cap Y)=I(X)+I(Y)$).
How to prove it?
&...
- 6,237
0
votes
1
answer
88
views
Proving finite presentation [closed]
Let $R$ be an integral domain, $S$ be a finitely presented $R$ algebra. Then for a flat $R$ module $M$ which is also a finitely generated $S$ module I need to show that $M \otimes_{R}T$ is a fintely ...
- 56