All Questions
Tagged with galois-cohomology etale-cohomology
24 questions
2
votes
0
answers
70
views
Finite dimensionality of Galois cohomology
Let $K_S$ denote the maximal extension of $\mathbb{Q}$, unramified outside a finite set of primes $S$, and let $G_S$ denote the Galois group of $K_S/\mathbb{Q}$.
It is known that for any finitely ...
- 2,907
1
vote
1
answer
678
views
Cohomology with coefficients in $\mu_\infty$
I'm encountering a lot of problems when dealing with the root of unity sheaf $\mu_\infty := \mathrm{colim}_n\mu_n$.
Let $X$ be a smooth geometrically integral variety over a number field $k$. Although ...
- 895
3
votes
0
answers
152
views
Obtaining an exact sequence of Galois modules via derived functors
This question has two parts, the first part will be to obtain the desired exact sequence while the second will be to study it in the corresponding derived category and try to obtain it from there.
Let ...
- 895
3
votes
2
answers
394
views
Is it true that $ H^{2r} ( X , \, \mathbb{Q}_{ \ell } (r) ) \simeq H^{2r} ( \overline{X} , \, \mathbb{Q}_{ \ell } (r) )^G $?
Let $ k $ be a field and let $ X $ be a smooth projective variety over $ k $ of dimension $ d $.
We denote by $ \overline{X} = X \times_k \overline{k} \ $ the base change of $ X $ to the algebraic ...
- 595
8
votes
2
answers
2k
views
The Mumford-Tate conjecture
The Mumford-Tate conjecture asserts that, via the Betti-étale comparison isomorphism, and for any smooth projective variety $ X $, over a number field $ K $, the $ \mathbb{Q}_{ \ell } $-linear ...
- 595
11
votes
1
answer
3k
views
What is known about the cohomological dimension of algebraic number fields?
What is the cohomological dimension of algebraic number fields like $\Bbb{Q}$, $\Bbb{Q}[i]$, $\Bbb{Q}[\sqrt{3}]$ or similar? I'm interested in computing the cohomological dimension of $\Bbb{A}^1_k$ ...
- 311
2
votes
0
answers
158
views
Fundamental Group of small Zariski open set
Let $Y$ be an integral affine variety over $\mathbb{C}$ and $K$ be its function field. How to find a sufficiently small Zariski open set of $Y$ such that it is isomorphic to $K(\pi,1)$? Here $\pi$ is ...
- 677
3
votes
1
answer
276
views
Semi-simple Galois actions on étale cohomology
Assume that semi-simplicity of the Galois action on $\ell$-adic cohomology of all smooth projective varieties over finite fields, were known.
Can one deduce that the Galois action on $\ell$-adic ...
user113393
6
votes
0
answers
364
views
Galois invariants in étale cohomology
Suppose $X$ is a smooth projective variety over a field $k$, with separable closure $\overline{k}$, Galois group $G$, and let $\overline{X}$ be $X_{\overline{k}}$.
Do we have
$$(H^j(\overline{X},\...
user120812
2
votes
0
answers
264
views
etale cohomology of tori
Let $k$ be an algebraically closed field.
Let $A$ be a strictly henselian local ring which is a $k$-algebra.
Let $T$ a torus over $k((t))$.
Can we compute $H^{1}(A((t)),T)$?
- 3,472
9
votes
3
answers
2k
views
Etale cohomology with coefficients in $\mathbb{Q}$
Let $X$ be a smooth variety of a field $k$. Then is
$$H_{et}^i(X, \mathbb{Q}) = 0$$
for all $i > 0$?
The result is true for $i=1$. This follows from the same argument given for $\mathbb{Z}$-...
- 21.1k
3
votes
0
answers
243
views
Relation between Galois and etale cohomologies
Let $D$ be the ring of integers of a number field $F$.
Let $X=\mathrm{Spec} ~D$, and let $\pi$ be the etale fundamental group of $X$.
There are natural maps from $H^i(\pi, \mathbf{Z}/n)$ to $H^i_{...
- 367
8
votes
0
answers
157
views
defining Selmer groups using étale cohomology
Concerning http://swc.math.arizona.edu/aws/1999/99RubinES.pdf, especially section I.5:
Can one define the Selmer groups and the unramified cohomology groups as étale
cohomology groups of certain ...
user19475
3
votes
1
answer
620
views
Galois cohomology of a non-abelian group over a function field
Let $k$ be an algebraically closed field, and $X$ a connected smooth projective curve over $X$. Let $F$ be the function field of $k$. Let $G$ be an algebraic group over $k$ (assume that it is smooth, ...
- 5,562
8
votes
1
answer
426
views
When does the continuous Galois(=etale) cohomology of fields coincide with the naive one? Often true by the Bloch-Kato conjecture?
For a field $F$ I am interested in its $l$-adic (Galois=\'etale) cohomology; here $l$ is a prime distinct from the characteristic of $F$ (for simplicity one may assume that the latter is $0$).
For $...
- 16.9k
8
votes
1
answer
569
views
Continuity of l-adic cohomology: is the cohomology of the generic point isomorphic to the completion of the limit of cohomology of open subvarieties?
Let $X$ be a variety over an algebraically closed field $k$. Denote by $\eta$ its generic point; it is the inverse limit of the open subvarieties $X_i$ of $X$. It is well known that the etale ...
- 16.9k
3
votes
2
answers
651
views
Galois cohomology of the field of Laurent series
Let $k$ a separably closed field. Do we have that $k((t))$ is of cohomological dimension one?
- 3,472
1
vote
0
answers
170
views
Galois cohomology of generic points of formal completions (of components of a hypercovering of the subvariety): examples or general statements?
Let $Y$ be a closed smooth subvariety in a (smooth) affine variety $X$. What can one say about the etale cohomology of the generic points of the formal completion of $X$ along $Y$ i.e. about the ...
- 16.9k
0
votes
0
answers
324
views
Ordered Cech(-like) complexes that compute etale cohomology (of fields!)
It is well known (cf. Equivalence of ordered and unordered cech cohomology. ) that for 'usual' topologies one can compute the cohomology of sheaves either using unordered Cech complexes or ordered ...
- 16.9k
2
votes
1
answer
585
views
Additive form of Hilbert 90 for schemes?
First, I am by no means well-versed on cohomology so I apologize if this is too elementary.
I have been going through some basics of etale cohomology, with my ultimate goal being an understanding of ...
- 801
8
votes
1
answer
1k
views
Galois descent for K-groups (or for étale cohomology groups)
Let $F/K$ be a Galois extension of number fields with Galois group $G$. Let $\mathcal{O}_F$ and $\mathcal{O}_K$ be the associated rings of integers, and let $n\geq 1$.
When is
$$
K_{2n-1}(\...
- 13k
32
votes
1
answer
3k
views
How is etale cohomology of integer rings related to Galois cohomology?
In the paper of Bloch and Kato in the Grothendieck Festschrift, and some other papers relating to the Bloch-Kato conjecture and the ETNC, the cohomology groups
$H^i_{\mathrm{et}}(\operatorname{Spec} ...
- 37k
5
votes
1
answer
1k
views
Galois cohomology groups given by étale cohomology
What are cases when Galois cohomology groups are given by étale cohomology?
Example: $S = Spec(K)$ the spectrum of a field, $F \in Sh(K)$, then $H^p(K, F) = H^p(G_K, F_{\bar{K}})$.
What if $G = \...
- 417
25
votes
1
answer
3k
views
Are all Galois cohomology groups also étale cohomology groups?
Let $K$ be a field and $K^s$ a separable closure of $K$, and let $\mathcal{F}$ be a sheaf on $\mathrm{Spec}(K)$ (in the étale topology).
By Grothendieck's Galois Theory, we have the isomorphism
$$H_{...
- 5,530