All Questions
6,057 questions
2
votes
1
answer
350
views
Base change of normalization map and scheme-theoretic surjectivity
Let $C$ be an affine, integral curve and $f: \widetilde{C} \to C$ be its normalization. Let $g:D \to C$ be a finite, affine, surjective morphism (note $D$ need not be reduced, but can assume ...
CommunityBot
- 1
5
votes
1
answer
209
views
If a finite poset supports a Cohen-Macaulay ASL, how far can it be from Cohen-Macaulay?
By the fundamental work of De Concini, Eisenbud, and Procesi, an algebra with straightening law (ASL) must be Cohen-Macaulay if it is built on a Cohen-Macaulay poset. I would like to understand the ...
- 2,851
2
votes
0
answers
194
views
if $I$ is finitely presented nilpotent and $M/IM$ is finitely presented, then $M$ is finitely presented
Let $R$ be a commutative ring, and let $I \subseteq R$ be a nilpotent ideal. Let moreover $M$ be an $R$-module, and let $IM$ be the submodule generated by the products $xm$ with $x \in I$ and $m \in M$...
- 2,079
2
votes
0
answers
108
views
Can an abelian group have multiple different actions of $\mathbb{Z}_p$?
This is perhaps a trivial question, but I've asked a few colleagues and they couldn't answer. For a given abelian group $M$, is it possible to have several different actions of the ring of $p$-adic ...
- 21
1
vote
0
answers
70
views
Constructive factorisation of null-homology map through acyclic complex
Let $f: C \rightarrow D$ be a maps of chain complexes on an idempotent complete additive category with all kernel or cokernel (or chain complexes on abelian category).
If $f$ induces a null map in ...
- 69
28
votes
1
answer
1k
views
What are retracts of polynomial rings?
Is there a known example of a ring endomorphism $f: \mathbb{Z}[x_1, \ldots, x_n] \to \mathbb{Z}[x_1, \ldots, x_n]$ such that $f \circ f = f$ but whose image is not isomorphic to a polynomial ring?
...
- 16.9k
1
vote
1
answer
281
views
Functoriality of Ext-functor
Let $X$ be a normal, integral variety and $U \subset X$ an open subset such that the complement of $U$ is of codimension at least $2$. Let $F$ be a coherent sheaf on $X$ such that $\mathcal{E}xt^1_U(F|...
- 38k
0
votes
0
answers
268
views
Non-Noetherian local ring with nilpotent maximal ideal
Browsing through my notes on Artin rings, I have realized that I don't know an example for this and I wasn't able to google anything relevant.
What is an example for a commutative non-Noetherian ...
- 7,127
6
votes
1
answer
415
views
Example of a projective module with non-superfluous radical
Let $R$ be a ring with unit. A submodule $N$ of an $R$-module $M$ is called superfluous if the only sumbodule $T$ of $M$ for which $N+T = M$ is $M$ itself.
It is shown, for example, in
[1] F. W....
- 315
2
votes
1
answer
770
views
Finiteness conditions and Veronese subrings
Consider a commutative group $G$ of finite type, a subgroup of finite index $H\subseteq G$, a noetherian commutative ring $A$, and a $G$-graded $A$-algebra $R=\bigoplus_{g\in G}R_g$ with no zero-...
1
vote
1
answer
267
views
Second summand to make projective module free
Suppose there's a projective $R$-module $P$ (non-free). We know that there is another $R$-module $M$ such that $P\oplus M$ is free over $R$. Is there a way to write down such an $M$ in terms of $P$?
...
- 23k
1
vote
1
answer
487
views
A question about Dedekind schemes and proper morphisms
The following is Exercise 15.3 of Görtz-Wedhorn Algebraic Geometry I:
Let $S$ be a Dedekind scheme with function field $K$ and let $f: X\to S$ be a proper morphism of schemes. Then the canonical map $...
- 7,746
47
votes
1
answer
1k
views
Summing infinitely many infinitesimally small variables makes sense in algebra
There is an identity $e^x=\lim_{n\to \infty} (1+x/n)^n$, and I always thought it is a purely analytic statement. But then I discovered its curious interpretation in pure algebra:
Consider the ring of ...
- 17k
1
vote
0
answers
78
views
When minimal prime ideals are maximal with respect to not containing an element
Let $\{ P_i \}$ be the set of all minimal prime ideals of a commutative ring $R $. Is there any conditions on $R $ under which there exists an element $x\in R $ such that $P_i $ is an ideal of $R $ ...
- 5,429
3
votes
2
answers
417
views
Norm on tensor product of fields
Let $F$ be an algebraically closed field of characteristic $p$ equipped with an absolute value $|\cdot|:F \rightarrow \mathbb{R}_{\ge 0}$ with respect to which $F$ is complete.
Define $|\cdot|_{prod}$...
- 1,144
3
votes
1
answer
836
views
Solving multilinear equations
Let $N=\{1,2,\ldots,n\}$. Suppose we are given $n$ equations, with each equation taking the form $\sum_{A\subseteq N}\left(c_A \prod_{i\in A}x_i \right) = 0$, where each $c_A$ is a real number ...
- 16.9k
13
votes
2
answers
4k
views
Interpretation of multiplicity of a point
Let $x$ be a (closed) point on an algebraic variety $X$ (of dimension $n$) defined over an algebraically closed field $k$. What is the multiplicity $mult_x(X)$, and how to compute it?
While having a ...
- 7,722
7
votes
2
answers
383
views
Counting nilpotent self-maps of $\{1,\dots,n\}$ with image of a given cardinal
Let $\mathcal{C}_n$ be the monoid of self-maps $\alpha$ of $\{1\dots,n\}$ that are order-preserving ($\forall x,y$, $x\le y$ $\Rightarrow$ $\alpha(x)\le\alpha(y)$ and decreasing ($\forall x$, $\alpha(...
- 63.9k
6
votes
1
answer
275
views
Algebraic geometry additionally equipped with field automorphism operation
I am looking for some facts on theory, which is essentially algebraic geometry but with field automorphisms added as 'basic' operations. (Precisely, I mean universal algebraic geometry for (universal) ...
- 63.9k
4
votes
3
answers
1k
views
Polya's theory of counting and commutative algebra
Do you know if there exist algebraic studies of the ring of the power series which emerge when using the theory of Polya for enumeration of sets with certain symmetries? For instance if some ideals ...
- 4,713
21
votes
5
answers
1k
views
Computation of fraction field of formal series over the integers
What is the fraction field $K$ of the domain $\mathbb Z[[X]]$?
It is strictly smaller than the field of Laurent series $L=\operatorname {Frac}\mathbb Q[[X]]$, since $\sum_{i\geq 0}\frac {X^i}{i!}\in ...
- 149k
5
votes
1
answer
223
views
Intrinsic characterisation of a class of rings
This may be well known, but I was unable to find an answer browsing literature. Let us temporarily call a commutative (unital) ring $R$ an O-ring if there exists an integer $n \ge 1$, a local field of ...
- 6,224
7
votes
2
answers
327
views
How to understand the "boundary" of subscheme, as defined in "An elementary characterisation of Krull dimension"
In An elementary characterisation of Krull dimension and A short proof for the Krull dimension of a polynomial ring, Coquand, Lombardi, and Roy give an elementary characterization of Krull dimension, ...
- 56.9k
1
vote
1
answer
100
views
category of non-welldefined linear maps
I was wondering whether the following category already has been used somewhere and whether it already has been named.
Let us fix a field $k$ (or more generally a ring). An object is just a $k$-vector ...
- 11.1k
5
votes
0
answers
2k
views
Saturated ideals in computational algebra
Let $R$ a commutative ring with one and $I, J \triangleleft R$ two ideals.
The saturated ideal $I^{sat}_J$ with respect $J$ is the ideal
$$(I : J^\infty )= \cup_{n \geq 1} (I:J^n)$$
where $(I:J^n)= \{...
- 6,048
11
votes
2
answers
1k
views
Valuations on tensor products
Let $A$ be a commutative ring, $B$ (resp. $C$) be a commutative $A$-algebra endowed with a valuation $v$ (resp. $w$), not necessarily of rank 1. Assume that $v$ and $w$ induce equivalent valuations on ...
- 1,144
3
votes
1
answer
182
views
Can one turn finite-dimensional vector subspaces into a cancellative semigroup?
Let $V$ be a vector space over some field and let ${\rm Fin}\,V$ be the family of all finite-dimensional subspaces of $V$. Is it possible to turn ${\rm Fin}\,V$ into an commutative cancellative ...
- 63.9k
1
vote
1
answer
208
views
On a condition on ideals viwed as a Zariski open condition on co-tangent space
Let $(R, \mathfrak m,k)$ be a Noetherian local ring such that the residue field $k$ is infinite. Let $n=\mu(\mathfrak m)$. Then $n=\dim_k(\mathfrak m/\mathfrak m^2)$ . By fixing $x_1,...,x_n \in \...
- 30.5k
16
votes
3
answers
797
views
For which rings R is SL_n(R) generated by its n-1 fundamental copies of SL_2(R)?
By "fundamental copies" of $SL_2(R)$ in $SL_n(R)$, I mean those embedded along the diagonal (for instance, if $n=3$, those are the upper left and lower right corner copies of $SL_2(R)$ embedded in $...
CommunityBot
- 1
2
votes
0
answers
96
views
Non-commutative version of the order dimension of a poset
I view the order dimension of a poset $P$ as an inherently commutative notion. On the one hand, it can be defined via realizers, which I find fairly intuitive from an order-theoretic viewpoint. On the ...
- 63.9k
3
votes
1
answer
208
views
Faithful flatness of left adjoint to almostification of algebras
I have been reading Bhatt's notes on perfectoid spaces and I have stumbled upon a fact whose proof I am unable to understand. Specifically, in Remark 4.2.8 Bhatt describes the functor $A\mapsto A_{!!}$...
- 28.2k
3
votes
0
answers
110
views
Is this a lattice?
Let $R$ be a locally compact ring (commutative with unit) and let $D\subset R$ be a discrete cocompact subring (cocompact means the additive group $R/D$ is compact). Let $G$ be a semisimple linear ...
- 63.9k
3
votes
0
answers
98
views
Hales' generalization of the stacked bases theorem (seeking a proof)
In his paper Analogues of the stacked bases theorem, published in the proceedings of a 1976 conference, A.W. Hales claimed some interesting generalizations of the stacked bases theorem for abelian ...
- 2,992
3
votes
0
answers
271
views
Explanation for devissage argument
Let $K$ be a local field of characteristic $0$ with the ring of integers $\mathcal{O}_K$ and uniformizer $\pi$. Let $k$ be the residue field of $K$ with $\text{card}(k)=q$. Let $\mathcal{O}_\mathcal{E}...
- 101
5
votes
3
answers
577
views
Is the class of power-associative binars finitely axiomatizable?
A binar is simply a set $S$ equipped with a single binary operation $*$. A power-associative binar is a binar where the subalgebra generated by a single element is associative. Equivalently, they can ...
- 14.6k
4
votes
1
answer
764
views
Further developments of Cartier–Gabriel–Kostant–Milnor–Moore Structure Theorem for cocommutative Hopf algebras
A very well-known theorem in Hopf algebra theory (see, for example, Lorenz - A tour of representation theory or the EGNO book (Etingof, Gelaki, Nikshych, and Ostrik - Tensor categories)) states that ...
- 718
6
votes
1
answer
202
views
Identities of finite inverse semigroups
An inverse semigroup is an algebra with two operations: binary $\cdot$ and unary $^{-1}$ such that $\cdot$ is associative and $xx^{-1}x=x, xx^{-1}yy^{-1}=yy^{-1}xx^{-1}$. The Brandt semigroup with 1, $...
- 38.6k
3
votes
1
answer
332
views
Algebraic vector bundles on the punctured spectrum: an exact reference for a result
Let $(R, \mathfrak m)$ be a Noetherian local ring of depth at least $2$. Let $X=Spec(R)$ denote the affine- scheme with structure sheaf $\mathcal O_X$ and $U=Spec(R)\setminus \{\mathfrak m\}$ be the ...
- 30.5k
2
votes
1
answer
294
views
The local flatness criterion
I am self studying the book "Commutative Ring Theory" by H. Matsumura. The main theorem of section 22 is the theorem 22.3, which characterizes flatness of a module $M$ over any ring $A$. The (part of ...
- 1,356
7
votes
2
answers
606
views
convergence in $\hat{\mathbb{Z}}$, modulo prime power
The following problem appears in Lenstra's Galois Theory for Schemes (p 14, Ex 1.16).
Let $b\in\mathbb Z_{\ge0}$. Define the sequence $(a_n)_{n=0}^\infty$
by $a_0=b, a_{n+1}=2^{a_n}$. Prove that $...
- 63.9k
2
votes
0
answers
252
views
Is there a name for commutative algebras over a field $k$ whose residue class fields have finite dimension over $k$?
Let $k$ be a field and let $A$ be a (commutative) $k$-algebra. Assume that for every maximal ideal $P \subseteq A$ the residue class field $A/P$ has finite dimension as a $k$-vector space.
Is ...
- 63.9k
28
votes
5
answers
4k
views
Does Smith normal form imply PID?
Let $R$ be a nonzero commutative ring with $1$, such that all finite matrices over $R$ have a Smith normal form. Does it follow that $R$ is a principal ideal domain?
If this fails, suppose we ...
- 2,992
3
votes
1
answer
368
views
Do height $h$ prime ideals in regular local rings contain regular sequences of length $h$?
Let $R$ be a regular local ring and let $P$ be a prime ideal of height $h$ in $R$.
Is it always the case that $P$ contains a regular sequence of lenght $h$?
This is clear if $h$ is $0,1$ or $\dim R$.
...
- 46
4
votes
0
answers
62
views
Is there a $3\times 3$ matrix over a Dedekind domain not similar to a matrix with zero top right entry?
Let $R=\mathbb{Z}[\sqrt{-5}]$, which is well known to be a Dedekind
domain but not a PID. Let $\mathrm{M}_{3}(R)$ be the set of $3\times3$
matrices over $R$. Does there exist a matrix $A\in\mathrm{M}_{...
- 3,823
8
votes
1
answer
285
views
Degrees of syzygies of points in $\mathbb P^2$
Let $X$ be a collection of points in $\mathbb P^2$ over the complex numbers. Let $I_X$ be the defining ideal. I am interested in knowing when:
The syzygies of $I_X$ contains no linear forms. Since ...
- 4,713
5
votes
0
answers
561
views
Brauer groups of a local ring and of its residue field
This is a question of DeMeyer (see the last paragraph of [1]):
What's an example of a local ring $A$ with residue field $k$ such that the restriction map on Brauer groups $\varphi : \operatorname{...
- 2,017
5
votes
1
answer
349
views
A generalization of integral Poincaré duality
In this paper, Felix, Halperin and Thomas define the notion of a Gorenstein space over a field $\mathbb{k}$:
An augmented differential graded algebra $R$ over $\mathbb{k}$ is Gorenstein if $\text{Ext}...
CommunityBot
- 1
1
vote
0
answers
28
views
Modified straightline complexity of almost square of sums
Assume every linear operation (such as inner product with constant vector) can be performed in one step and multiplication by variables (quadratic operation) can be performed in one step.
We know the ...
CommunityBot
- 1
28
votes
1
answer
2k
views
SOS polynomials with integer coefficients
A well known theorem of Polya and Szego says that every non-negative univariate polynomial $p(x)$ can be expressed as the sum of exactly two squares: $p(x) = (f(x))^2 + (g(x))^2$ for some $f, g$. ...
CommunityBot
- 1
5
votes
1
answer
303
views
faithful modules over a finite dimensional commutative algebra
Let $A$ be a commutative algebra over a field $k$ which is finite dimensional as a vector space over $k$. Let $M$ be a faithful $A$-module. Does it follow that $dim_k(M)\geq dim_k(A)$?
- 30.5k