Questions tagged [brauer-groups]
Questions concerning Brauer groups of fields, rings, varieties, schemes or more general ringed spaces, invariants associated to Brauer classes such as index and period.
99
questions
3
votes
1
answer
205
views
Unitary involutions on a simple central algebra after a scalar extension
$\DeclareMathOperator{id}{id}$
Let $L/K$ be a quadratic separable extension of fields. Let $A$ be a central simple algebra over $L$ such that its norm $N_{L/K}(A)$ splits. Then we know that there ...
- 93
1
vote
0
answers
114
views
The Brauer group of the function field of a proper curve
Let $X$ be a smooth proper geometrically connected curve over a number field $k$, and let $k(X)$ denote its field of rational functions, i.e., its function field. Then the (cohomological) Brauer group ...
- 835
2
votes
0
answers
103
views
Results concerning surjectivity of Brauer groups
Are there known cases of a morphism of smooth geometrically connected curves $f: X \rightarrow Y$ over a number field $k$ (to be specific) that would give rise to a surjective restriction map $f^*:\...
- 835
5
votes
1
answer
184
views
Torus gerbes over curves
Setup: Let $k$ be an algebraically closed field. Let $C$ be a smooth connected curve over $k$. Let $K(C)$ be the function field of $C$.
Tsen's Theorem implies that every $\mathbb{G}_m$-gerbe over $K(C)...
- 51
1
vote
0
answers
162
views
Vanishing of the local étale cohomology sheaf (?)
Let $X$ be a locally noetherian regular scheme, and let $Z$ be a closed subscheme of $X$ whose codimension $d > 0$ at every point.
Let $U$ be the complement of $Z$ in $X$.
For a sheaf $\mathscr{F}$ ...
- 185
1
vote
1
answer
221
views
Counterexample to purity of Brauer group for curves
The purity of Brauer group states that for a smooth (quasi-)projective variety $X$ over a field $k$, removing a closed subscheme $Z \subset X$ of codimension at least $2$ ensures that the restriction ...
- 835
4
votes
0
answers
138
views
A map between Brauer groups
Let $R$ be a henselian dvr over $\mathbb{C}$ and $A$ be a flat $R$-algebra of finite type. Suppose $\hat{R}$ is the completion of $R$ and $\hat{A}:=A\otimes_R \hat{R}$. For an ideal $I\subseteq A$, ...
- 156
2
votes
0
answers
111
views
Why is Maycock's Brauer group of graded C*-algebras connected while Moutuou's is not?
In her thesis The Brauer Group of Graded Continuous Trace $C^\ast$-Algebras (cf. Proposition 3.4), Ellen Maycock described the Brauer group of graded continuous trace $C^\ast$-algebras with spectrum a ...
- 9,855
5
votes
1
answer
655
views
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 ...
- 109
3
votes
0
answers
241
views
Can moonshine be explained by the $q$-expansion map $\mathrm{tmf} \to KU[[q]]$?
If we form an equivariant version of both sides of the $q$-expansion map $\mathrm{tmf} \to KU[[q]]$ (with everything being $p$-completed), should the equivariant $q$-expansion map should have ...
- 428
2
votes
0
answers
125
views
Field of definition for a representation and Brauer group
I wonder if the following logic is true. And if it is true, how to prove it?
Let $G$ be a profinite group, $\rho:G\to GL_n(k)$ a continuous representation, where $k$ is an algebraically closed field ...
- 363
1
vote
0
answers
101
views
Galois extension of $ C_{k} $ field and irreducible polynomial over $ C_{k} $ field
A field is called a $ C_{k}$- field if every form i.e. every homogeneous polynomial of degree $ d $ in $ n > d^{k} $ indeterminates has a nontrivial zero. We know that any finite field is a $ C_{...
- 693
5
votes
1
answer
220
views
Countably many isomorphism classes of reductive groups over a field with countable Brauer and Witt groups
Assume a field has a countable Brauer group and a countable Witt group. Are there countably many isomorphism classes of reductive groups over it?
- 51
9
votes
1
answer
659
views
Software for detecting Brauer-Manin obstructions?
In the context of another MO question, the following question arose: Does there exist any software for detecting Brauer–Manin obstructions to the existence of integer solutions to a single polynomial ...
- 72.6k
3
votes
0
answers
150
views
The Brauer group and the second Galois cohomology group
I know that for any finite Galois extension K of $\mathbb{Q}$ the group $H^2(Gal(K/\mathbb{Q}),K^*)$ is isomorphic to the Brauer group $Br(K/\mathbb{Q})$. The isomorphism goes as follows: to a 2-...
- 483
1
vote
0
answers
66
views
Would the iterated finite abelian descent obstruction equality hold for curves?
Let $X$ be a smooth projective geometrically integral variety over a number field $k$. We begin with some established notions in the theory of descent obstruction to the local-global principle, ...
- 835
8
votes
1
answer
589
views
Subspaces of $ A_{n}(\mathbb {Q})$ in which all nonzero matrices are invertible
Let $A_{n}(\mathbb{Q}) $ denote the $n$ times $n$ skew symmetric matrices over the rational number field. Let $N$ be a subspace of $A_{n}(\mathbb{Q}) $.
If all the non-zero matrices in $N$ are ...
- 693
3
votes
0
answers
177
views
Central division algebras over $ \mathbb{Q} $
Quaternions over $ \mathbb{Q} $ are an example of a Central Division algebra over $\mathbb{Q} $ for which the basis elements $\{ i,j,ij \} $ other than $1$ are represented by skew-symmetric matrices ...
- 693
2
votes
0
answers
141
views
Understanding Sha through $K_2$
Consider the following setup, to keep things easy: let $F$ be a number field with ring of integers $A$. Let $E$ be an elliptic curve over $F$ with Neron model $N$ over $A$. Let $Sha(E)$ be the ...
- 12.8k
3
votes
1
answer
222
views
A local-to global principle for splitting of Azumaya algebras
Let $S$ be a finitely generated domain with the field of fractions $F.$ Let X be a smooth,
geometrically connected affine variety over $S.$ Let $A$ be an Azumaya algebra over $X.$
Assume that for all ...
- 33
3
votes
0
answers
91
views
Multiplication law in a central simple algebra of dimension 9 over a global field
Let $k$ be a global field, for example $k=\Bbb Q$.
Let $D$ denote the central simple algebra of dimension 9 over $k$ with given local invariants $i_v$.
Here $v$ runs over the set $\Omega_f(k)$ of ...
- 12.6k
3
votes
0
answers
89
views
Multiplication law in a division algebra of dimension 9 over a non-archimedean local field
Let $k$ be a non-archimedean local field, for example, a $p$-adic field (a finite extension of the filed ${\Bbb Q}_p$ of $p$-adic numbers).
It is well known that there is a canonical isomorphism
$${\...
- 12.6k
2
votes
0
answers
147
views
Interpretation of some maps involving cohomology groups
I've asked some questions on Math Stackexchange regarding areas around my research but I received very little success with responses, so I thought I might try to share some of my other problems here ...
- 835
8
votes
0
answers
625
views
An Azumaya algebra from a vector bundle, and a construction of Belov-Kanel and Kontsevich
Let $S/k$ be a scheme over a perfect field $k$ of characteristic $p>0$.
In Automorphisms of the Weyl Algebra, Belov-Kanel and Kontsevich write down the map
$$\alpha: H^0(\Omega^1_{S/k}/d\mathcal O) ...
- 1,340
4
votes
2
answers
458
views
Brauer group of $\mathbb{Z}_{(p)}$
This may be a well known result but I could not find it in the standard references. What is the Brauer group of the local ring $\mathbb{Z}_{(p)}$ (the ring of integers localized at $p$)?
- 81
7
votes
0
answers
213
views
Brauer group of the Henselization
Let $R$ be a Noetherian local ring and let $R^h$ be its Henselization. What can we say about the kernel and range of the map
$$
\operatorname{Br}(R) \rightarrow \operatorname{Br}(R^h)?
$$
Are there ...
- 81
1
vote
1
answer
118
views
$0$-th Galois cohomology with topological Milnor K-groups coefficients
In local class field theory, the reciprocity map is constructed by using the isomorphism ${\rm Br}(F)\simeq \mathbb{Q/Z}$, where $F$ is a local field and ${\rm Br}(F)$ is its Brauer group. The ...
- 479
3
votes
1
answer
632
views
Cohomological Brauer group vs classical
Let $X$ be a smooth scheme over $\mathbb{C}$.
A $O_X$-algebra $A$ is called Azumaya algebra on $X$
if locally it's ismorphic to matrix algebra: ie for
every $p \in X$ there exist open $U \subset X$ ...
- 4,966
7
votes
1
answer
452
views
Category of modules over an Azumaya algebra and the Brauer group
Let $k$ be a field, and let $\alpha \in \mathrm{Br}(k)$. Let $A$ be an Azumaya algebra representing $\alpha$. Then the category $A$–$\mathrm{mod}$ depends only on $\alpha$.
I would like to know ...
- 14.7k
3
votes
0
answers
173
views
Why doesn't the Manin obstruction work for quadratic forms?
The Manin obstruction explains why the Hasse principle doesn't work for non-quadratic forms. Let's write the notation first;
$V(\mathbb{Q})$ is variety for rational numbers.
$V(A_\mathbb{Q})$ is ...
- 39
6
votes
1
answer
232
views
Involution action on Brauer group of an abelian variety
Let $k$ be an algebraically closed field of characteristic $p>2$, let $A/k$ be an abelian variety. Let $\iota\colon A\to A, a\mapsto -a$ be the natural involution. Let $x\in\mathrm{Br}(A)[p]$ be a ...
user39380
5
votes
0
answers
411
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{...
- 1,967
15
votes
1
answer
705
views
Postnikov invariants of the Brauer 3-group
Given a commutative ring $k$ there is a bicategory with
algebras over $k$ as objects,
bimodules as morphisms,
bimodule homomorphisms as 2-morphisms.
This is a monoidal bicategory, since we can ...
- 20.6k
3
votes
1
answer
276
views
What is known about lower etale cohomology of unirational varieties?
Which information is currently known about $H^1_{et}(X,\mathbb{Z}_l)$ and $H^2_{et}(X,\mathbb{Z}_l)$, where $X$ is a smooth unirational variety over an algebraically closed field of finite ...
- 16.1k
4
votes
1
answer
269
views
Can base-change be non-surjective on Brauer groups?
Is there a finite-degree separable field extension $\mathbb{K} \subset \mathbb{L}$ such that the induced map on Brauer groups $\operatorname{Br}(\mathbb{K}) \to \operatorname{Br}(\mathbb{L})$ is not a ...
- 51.4k
5
votes
1
answer
257
views
Relation in Brauer group coming from trace form
Let $L/K$ be a cubic (or, more generally, odd-order) extension of fields of characteristic $0$. To every element $a \in L^\times$ we can associate the quadratic form
\begin{align*}
q_a : L &\to K \...
- 636
3
votes
1
answer
283
views
Are Azumaya algebras of trivial Brauer class isomorphic to $\mathcal{E}nd(\mathcal{H})$?
Let $X$ be a scheme, let $\mathcal{A}$ be a sheaf of locally free algebras on $X$. We say $\mathcal{A}$ is an azumaya algebra, if the natural map $$\mathcal{A}\otimes_{\mathcal{O}_X}\mathcal{A}^{opp}\...
user39380
6
votes
1
answer
404
views
Purity of Brauer group for stacks
Let $k$ be a field, let $X$ be a smooth quasi-projective $k$-variety, let $Z\subset X$ be a closed subscheme of codimension at least $2$, it is shown
that the restriction map $\mathrm{H}^2(X,\mathbb{...
user39380
5
votes
1
answer
330
views
Topological Hochschild homology of Azumaya algebra
Let $R$ be a commutative ring, let $A$ be an Azumaya algebra over $R$, does its topological Hochschild homology coincide with that of $R$? For example, let $\mathbb{H}$ be the quaternion algebra over ...
user39380
3
votes
0
answers
116
views
Topological Brauer group and sheaf of $C^{\infty}$-functions
Given a smooth manifold $X$; by definition the topological Brauer group $B(X)$ is the group of classes of Azumaya algebras over the sheaf $\mathcal{O}_X$ of $\mathbb{C}$-valued functions on $X$.
If ...
- 121
1
vote
0
answers
180
views
Killing a Brauer class by a flat projective morphism
Let $X$ be a Noetherian scheme and $\beta \in H^2(X, \mathbb{G}_m)$ that is the image of some $\alpha \in H^1(X, \mathrm{PGL}_{n + 1})$ for some $n \ge 0$, so $\beta$ is a class in the Brauer group of ...
2
votes
0
answers
276
views
Azumaya algebra and central separable algebra over local rings
let be $R$ a commutative ring
Auslander and Goldman define a central separabl $R$-algebra $A$ as a faithful $R$-module with $Z(A) = R$ , and such that $A$ is a projective $A\otimes_R A^{\mathrm{op}}$-...
- 121
2
votes
0
answers
246
views
Brauer groups and del Pezzo surfaces
Let $k$ be a field of characteristic $0$ und let $X$ be a del Pezzo surface over $k$. Note that $X$ may not have points.
Let us consider $N:=\ker(\mathrm{Br}(k) \rightarrow \mathrm{Br}(k(X))$.
...
- 1,387
3
votes
0
answers
62
views
Splitting of central simple algebras in the Schur subgroup over residue fields of places
Recall that a valuation domain of a field extension $K/k$ is a $k$-subalgebra $V$ of $K$ not equal to $K$ such that for every $a\in K$ at least one of $a$ and $a^{-1}$ is in $V$.
A place of $K/...
2
votes
0
answers
222
views
Brauer Group of a nodal curve
What is known about the Brauer Group of a Nodal curve (complete integral curve) over $k$ with singularity as ordinary double point?
Is it trivial if $k$ algebraically closed?
- 137
9
votes
2
answers
599
views
On a morphism from the Brauer group to the Picard group
Suppose that $k$ is a commutative ring and that $A$ is an Azumaya
$k$-algebra. Then there is a well-known morphism from $Aut_{Alg_k}(A)$, the group of algebra automorphisms, to the Picard group $Pic(k)...
- 49.6k
4
votes
0
answers
207
views
Brauer groups over local fields
Let X be a smooth projective variety over a local field of characteristic $(0,p)$. The Brauer group of X is a torsion group whose $l$-part is of cofinite type of some corank.
Is it know that the $l$-...
- 1,403
13
votes
1
answer
1k
views
Explicit examples of Azumaya algebras
I'm trying to understand the Brauer group of a scheme better. I know how to compute $\text{Br}(X)$ as an abstract group in some cases, but don't have a good idea of what the individual Azumaya ...
- 5,112
3
votes
1
answer
241
views
Are local fields $C_{2}$?
We say that a field $K$ is $C_{m}$ if it satisfies the following property: for every positive integer $n$ and every sequence of positive integers $(d_{1},\dotsc,d_{r})$ satisfying $d_{1}^{m} + \dotsb +...
- 1,967
12
votes
3
answers
1k
views
Brauer group of a curve over non-algebraically closed field
It is a famous consequence of Tsen's theorem that a smooth curve over an algebraically closed field has trivial Brauer group. But what about curves over non algebraically closed fields?
Let us fix a ...
- 793