All Questions
6,057 questions
17
votes
1
answer
555
views
Does every commutative variety of algebras have a cogenerator?
By a commutative variety $\mathcal{V}$ I mean a classical variety of algebras for some $(\Sigma,E)$, such that each pair of operations in $\Sigma$ commutes.
Equivalently (i) every interpretation of ...
- 63.9k
16
votes
4
answers
3k
views
Cardinal of maximal linearly independent subsets of a free module
Is it true that the cardinality of every maximal linearly independent subset of a finitely generated free module $A^{n}$ is equal to $n$ (not just at most $n$, but in fact $n$)? Here $A$ is a nonzero ...
- 63.9k
20
votes
4
answers
9k
views
For which $c$ is $\mathbb{Z}[\sqrt{c}]$ a unique factorization domain? a Euclidean domain?
Let $c$ be an integer, not necessarily positive and $|c|$ not a square. Let $\mathbb{Z}[\sqrt{c}]$ be the set of complex numbers $$a+b\sqrt{c}, \quad a, b\in \mathbb{Z},$$
which form a subring of the ...
- 4,713
18
votes
1
answer
987
views
Why does $E\otimes_KE\cong EG$ imply that Galois theory works?
This is a part of statement in the book I do not fully appreciate. Suppose $E/K$ is Galois extension and $G$ the Galois group of $E/K$. $E[G]$ is the group ring formed by finite group $G$.
"It is ...
- 63.9k
8
votes
2
answers
425
views
Dimension of commutative subalgebras of a central simple algebra
let $k$ be a field, and let $A$ be a central simple $k$-algebra over $k$.
What is the maximal dimension of a commutative $k$-subalgebra of $A$?
If $A=M_r(D)$, where $D$ is a central division $k$-...
CommunityBot
- 1
6
votes
2
answers
454
views
Approximating ring maps of finite Tor-dimension
Let $R$ be a commutative ring, and let $S$ be a finitely presented $R$-algebra of finite Tor-dimension over $R$. Can $R \to S$ be realized as the base change, along some ring map $R_0 \to R$, of a ...
- 1,144
38
votes
1
answer
10k
views
Infinite tensor products
Let $A$ be a commutative ring and $M_i, i \in I$ be a infinite family of $A$-modules. Define their tensor product $\bigotimes_{i \in I} M_i$ to be a representing object of the functor of multilinear ...
- 4,713
28
votes
1
answer
1k
views
Algebraic dependency over $\mathbb{F}_{2}$
Let $f_{1},f_{2},\ldots,f_{n}$ be $n$ polynomials in $\mathbb{F}_{2}[x_{1},x_{2},\ldots,x_{n}]$
such that $\forall a=(a_1,a_2,\ldots,a_n)\in\mathbb{F}_{2}^{n}$ we have $\forall i\in[n]:f_{i}(a)=a_{i}$....
- 63.9k
2
votes
1
answer
134
views
Jacobson semisimple varieties of commutative rings
Consider the following problem: given a variety of algebras (a class of algebras in a given algebraic signature defined by some set of equations), describe its semisimple subvarieties. That is, ...
- 14.6k
4
votes
0
answers
162
views
Prime/irreducible elements in certain (integral) ring extensions
The answer to this question says the following:
Let $R$ be a finitely generated $k$-algebra, where $k$ is a field.
If $p \in R$ is a prime element, then $p$ is a prime element in $\tilde{R}$, the ...
- 2,837
6
votes
1
answer
691
views
When are projective modules closed under highly-filtered colimits?
Let $R$ be a ring. Let $Mod(R)$ be the category of left $R$-modules, and let $Proj(R) \subseteq Mod(R)$ be the full subcategory of projective $R$-modules. Let's say that $R$-projectives are closed ...
- 15.6k
6
votes
0
answers
233
views
Morphism that is surjective on PID points is surjective on every Dedekind domain?
Let $f: X=\Bbb A^n \rightarrow Y=\Bbb A^m $ be a morphism between affine spaces over an algebraically closed field $k$. Assume $f: X(R) \rightarrow Y(R)$ is surjective for any PID $R$ over $k$, under ...
- 63.9k
13
votes
4
answers
4k
views
Size of a Groebner basis
I've spent some time recently looking at some Groebner bases for some specific ideals coming from problems in computer vision. The generators are not sparse, and they all have the same degree (...
- 4,425
10
votes
1
answer
1k
views
On Noetherian and Japanese rings
Let $R$ be a Noetherian ring all of whose local rings are Japanese. Is $R$ necessary Japanese?
- 1,823
4
votes
2
answers
459
views
Subrings of Jacobson rings
Suppose $A\subset B$ is an inclusion of commutative rings with $B$ Jacobson.
If $B$ is finitely generated as an algebra over $A$ does it follow that $A$ is Jacobson?
If $B$ is finitely generated as a ...
- 1,052
7
votes
1
answer
1k
views
Pushouts of noetherian rings
Does the category of noetherian commutative rings have pushouts?
Background: If $X/S$ is an abelian scheme, then the relative Picard functor $\mathrm{Pic}_{X/S}$ is only defined on the category of ...
- 63.1k
74
votes
1
answer
6k
views
$R$ is isomorphic to $R[X,Y]$, but not to $R[X]$
Is there a commutative ring $R$ with $R \cong R[X,Y]$ and $R \not\cong R[X]$?
This is a ring-theoretic analog of my previous question about abelian groups: In fact, in any algebraic category we may ...
- 63.9k
3
votes
1
answer
484
views
Harish-Chandra isomorphism for characteristic $p$
I am trying to understand the proof of Theorem 1 from this paper V. Kac and B. Weisfeiler (Indag. Math. 1976, DOI link).
Theorem 1. Let either $p\neq 2$ or $\varrho\in X(\mathscr{T})$. Then $\gamma(...
- 63.9k
11
votes
1
answer
475
views
Greatest common divisor in $\mathbb{F}_p[T]$ with powers of linear polynomials
Let $n>1$ and $p$ be an odd prime with $p-1 \mid n-1$ such that $p^k - 1 \mid n-1$ does not hold for any $k>1$. Notice that, since $p-1 \mid n-1$, we have $T^p - T \mid T^n-T$ in $\mathbb{F}_p[T]...
- 149k
1
vote
1
answer
326
views
Closed submonoid of $(\mathbb{C}^*)^n$
The answer of this question might be known but I was not able to find any answer. Let $n\geq 1$ and $S$ be a closed submonoid of $(\mathbb{C}^*)^n$, that is, a closed and stable by product subset of $(...
CommunityBot
- 1
6
votes
0
answers
267
views
Classification of one dimensional (non-commutative) formal group laws over $k[\epsilon]/(\epsilon^n)$
It's well-known that any one dimensional formal group law over a $\mathbb Q$-algebra or a reduced ring is commutative, but there are one dimensioal non-commutative formal group laws over rings like $k[...
- 63.9k
4
votes
4
answers
1k
views
Why do we choose the standard total order on the integers?
I understand why the set of natural numbers $\mathbb N = \{ 0, 1, 2, \cdots \}$ is equipped with a total order. Indeed, every monoid has a pre-order, where $$n' \succeq n \quad \mathrm{if~and~only~if} ...
- 63.9k
1
vote
0
answers
105
views
formal smoothness and McQuillan formal schemes
Let $k$ be an algebraically closed field, $A\rightarrow B$ be a continuous map of weakly admissible topological $k$-local algebras.
We assume that it is formally smooth and topologically of finite ...
- 3,472
0
votes
0
answers
154
views
Determinant of a special matrix in characteristic $p$
Let $K$ be a field of characteristic $p > 0$. Choose $p^i$ numbers of elements $c_1,\ldots,c_{p^i} \in K$ and consider the determinant $D$ of the following matrix$\colon$
\begin{pmatrix}\label{...
- 563
5
votes
0
answers
637
views
Unique product groups (and semigroups)
A group $G$ is called a u.p.-group (short for unique product group) if for all nonempty finite subsets $A,B\subseteq G$, there exists an element $g\in A \cdot B$ which can be uniquely written as a ...
- 63.9k
2
votes
0
answers
160
views
Making explicit the local structure theorem of étale maps in a very simple case
Making explicit the local structure theorem of étale maps in a very simple case.
First I recall the following items from Stacks.
Lemma 10.141.2. Any étale ring map is standard smooth. More precisely,...
- 9,650
4
votes
1
answer
307
views
Characterization of Archimedean linearly ordered monoids
In this question, it is shown that all Archimedean ordered groups are isomorphic to an ordered subgroup of $\mathbb R$. Additionally, it is shown that if such a group is complete, then it is ...
- 63.9k
2
votes
0
answers
240
views
Tensor product of fields 2
Let $K_1, K_2$ be finite field extensions of a field $k$.
Question: Is it true that $A=K_1 \otimes_k K_2$ is isomorphic to a product of group algebras over fields?
Question 2: In case the answer is ...
CommunityBot
- 1
2
votes
0
answers
91
views
Is the natural action of the monoid of endomorphisms is a complete invariant for group?
Let $\alpha$ and $\beta$ be actions of semigroups $A$ and $B$ on sets $X$ and $Y$ respectively. Recall that $\alpha$ and $\beta$ are called isomorphic if there exists an isomorphism $\phi$ between ...
- 63.9k
4
votes
2
answers
290
views
Free augmented algebras
What is the "correct" definition of a free augmented commutative algebra?
At least two definitions come to my mind:
Fix a commutative ring $k$. We need elements $\lambda_1,\dotsc,\lambda_n \in k$. ...
- 7,962
8
votes
1
answer
1k
views
Lattice-ordered commutative monoids
By a lattice-ordered monoid, I mean a structure $(A,0,{+},{\vee},{\wedge})$ such that $(A,0,{+})$ is a (not necessarily commutative) monoid, $(A,{\vee},{\wedge})$ is a lattice, and the two ...
- 63.9k
1
vote
0
answers
61
views
powers of linear forms in projections of complete intersections in codimension 3
Let $I\subset \mathbb{C}[x_0,x_1,x_2]=:A$ be a complete intersection, $I=(p_1,p_2,p_3)$, $p_i$ homogeneous all of the same degree d
for some $d>2$.
Let $l$ be a general linear form and let $J$ ...
- 41
5
votes
1
answer
525
views
Noether normalisation over $\mathbf{Z}$
Is there a Noether normalisation lemma for finitely generated (flat) algebras over $\mathbf{Z}$ (or more generally principal ideal domains)? It seems one can tensorise with the quotient field and then ...
CommunityBot
- 1
3
votes
0
answers
931
views
Special irreducible polynomials in $k[x,y]$
The following question I have asked in MSE, getting one comment.
Hopefully, it is ok to ask it here also.
Let $k$ be a field of characteristic zero, $n \in \mathbb{N}$.
Definitions:
(1) $0 \neq f \...
- 2,837
3
votes
0
answers
59
views
When flatness of $S$ over $R_i$ implies flatness of $S$ over the ring generated by $R_1,R_2$
The following question I have asked in MSE, but have not received an answer, so I ask it here; I really apologize if it is not suitable for MO.
Let $k$ be a field of characteristic zero and let $R_1,...
- 2,837
3
votes
1
answer
830
views
Is the support of a flat module generically flat?
Let $X$ be an affine, complex variety, $A$ be a $\mathbb{C}$-algebra (not necessarily noetherian) and $F_A$ is a coherent sheaf over $X \times \mbox{Spec}(A)$, flat over $\mbox{Spec}(A)$. Denote by $Y ...
- 63.9k
32
votes
5
answers
9k
views
How many binary operations are associative?
Let $X$ be a finite set of $n$ elements, and consider a binary operation $\odot: X \times X \rightarrow X$. There are $n^{n^2}$ such binary operations, as the $n \times n$ table entries can each
be ...
- 63.9k
-1
votes
1
answer
209
views
Flatness of certain quotient rings
Let $p,q \in \mathbb{C}[x,y]$ be two polynomials such that $p_xp_yq_xq_y \neq 0$
(namely, each partial derivative is non-zero).
Assume that the following four conditions are satisfied:
(1) $\frac{\...
- 28.7k
2
votes
1
answer
295
views
Completion and extension by scalars
Let $R\subset S$ be commutative rings, $I\trianglelefteq R$ an ideal and $M$ be an $R$-module. Suppose that
1) $R$ is Noetherian and $I$-adically complete.
2) $M$ is a finite $R$-module (hence $M$ ...
- 28.7k
10
votes
0
answers
314
views
How much do idempotent ultrafilters generate in terms of semigroups?
It is known that the set of ultrafilters on, say, the natural numbers $\mathbb{N}$, can naturally be endowed with the structure of a compact topological left semigroup (which fails to be anything ...
- 4,713
1
vote
1
answer
160
views
Map between localizations induces map on underlying modules for Zariski covering
While working through a proof of this paper,1 at the middle of page 45, the author's claim of a short exact sequence seems to amount to the following problem:
Let $A$ be a commutative ring and let $...
CommunityBot
- 1
4
votes
1
answer
518
views
Strictly totally ordered semigroups - Looking for references
Let $\mathfrak A = (A, \cdot)$ be a semigroup (written multiplicatively). We say that $\mathfrak A$ is linearly orderable if there exists a total order $\le$ on $A$ such that $ac < bc$ and $ca < ...
- 63.9k
18
votes
4
answers
4k
views
Flatness of normalization
Let $X$ be a noetherian integral scheme and let $f \colon X' \to X$ be the normalization morphism. It is known that, if non trivial, $f$ is never flat (see Liu, example 4.3.5).
What happens if we ...
- 20.5k
4
votes
0
answers
111
views
Name for "étale-essential" properties
A map of rings $f:A\to B$ is called "essentially $P$" if there exists some $A\to C\to B$ such that $A\to C$ has property $P$ and $C\to B$ is a localization, that is to say, a filtered colimit of ...
- 493
6
votes
0
answers
382
views
Is there a Dedekind domain which has infinite class group and is free of finite rank over a finite quotient PID?
Is there a Dedekind domain $B$ satisfying the following two conditions:
$B$ is an algebra over a finite quotient PID $A$ such that $B$ is free of finite rank as a module over $A$;
$B$ has infinite ...
- 3,823
4
votes
2
answers
393
views
Embedding a linearly ordered free monoid into a linearly ordered group
A linearly ordered (shortly, l.o.) monoid is a triple $\mathbb M = (M, \cdot, \le)$ for which $(M, \cdot)$ is a (multiplicatively written) monoid and $\le$ is a total order on $M$ such that $xy < ...
- 63.9k
7
votes
1
answer
266
views
Positive cone of a subgroup of $\mathbb{Z}^n$
This question sounds like it should be very well known, but for some reason I failed to find a decent answer anywhere. Let $G\subset\mathbb{Z}^n$ be a subgroup, and $G_+=G\cap\mathbb{Z}_{\ge0}^n$ be a ...
- 63.9k
1
vote
0
answers
126
views
Algebraic structures on graphs
There are many algebraic structures linked to graphs.
For example one can find zero divisor graphs $[1]$, $[2]$ and many other graphs.
Does there exist any survey paper which characterizes all the ...
- 63.9k
8
votes
2
answers
827
views
Which semigroups can be linearly ordered?
As usual I consider a semigroup to be a structure $(A, +)$ such that $+$ is an associative binary function over the set $A$. The notion of linearly-ordered semigroup corresponds to structures of the ...
- 63.9k
17
votes
12
answers
4k
views
Why semigroups could be important?
There is known a lot about the use of groups -- they just really appear a lot, and appear naturally. Is there any known nice use of semigroups in Maths to sort of prove they are indeed important in ...
- 6,237