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Questions tagged [galois-representations]

The term Galois representation is frequently used when the G-module is a vector space over a field or a free module over a ring, but can also be used as a synonym for G-module. The study of Galois modules for extensions of local or global fields is an important tool in number theory.

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Semisimplicity of induced representation of a irreducible representation

This question occurs when I read this one. Suppose $k$ is a field of char $0$ (not necessarily complex numbers, in my case it's $\mathbb Q_p$), and $G$ is a profinite group (in my case, $\operatorname{...
Richard's user avatar
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Are there known effective bounds on the number of semisimple Galois representations?

In continuation to my question here, are there known effective bounds on the total number of semisimple $p$-adic Galois representations unramified outside a finite set of primes $S$, of dimension $d$, ...
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Reference for Faltings' proof on finiteness of semisimple $d$-dimensional $p$-adic Galois representations

I'm looking for a reference to Faltings' proof concerning the finiteness of $d$-dimensional semisimple $p$-adic Galois representations. Specifically, the result states that there are only finitely ...
kindasorta's user avatar
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Finitely ramified global Galois representation is trivial on a subgroup of finite index in \mathcal{O}_K^*?

In Lawrence-Venkatesh, they state before lemma 2.8 that if a continuous $\eta:{\rm Gal}(\bar{K}/K)\to\mathbb{Q}_p^*$ is finitely ramified, where $K$ is a number field, then the induced homomorphism $\...
Phanpu's user avatar
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Evidence for the equivariant BSD conjecture with higher multiplicity

Let $E/\mathbb{Q}$ be an elliptic curve and let $\rho$ be an irreducible Artin representation. Let $K_\rho/\mathbb{Q}$ be the smallest Galois extension such that $\rho$ factors through $\mathrm{Gal}(...
J M T P's user avatar
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Question About Page 11 of Cornell, Silverman, Stevens, "Modular Forms and Fermat's Last Theorem"

Looking at page 11 of the text, consider a Galois representation $\rho: G_{\mathbb Q} \to \operatorname{GL}_2(A)$, where $A$ is a coefficient ring (i.e. complete Noetherian local ring with finite ...
Johnny Apple's user avatar
3 votes
1 answer
113 views

Depth of the filtration of higher ramification groups in the ramified case in Serre's modularity conjecture

I am studying Serre's paper "Sur les représentations modulaires de degré 2 de $\mathrm{Gal(\overline{\mathbb Q}/\mathbb Q)}$" and I have some questions about Serre's definition of "peu ...
Marta Sánchez Pavón's user avatar
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117 views

Automorphy of the twisted representation

The Artin reciprocity says that if $$ \chi: \operatorname{Gal}(K/\mathbb Q) \to \mathbb C $$ is a 1-dimensional representation of a finite Galois extension $K/ \mathbb Q$, then it corresponds to a ...
LWW's user avatar
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Rational isogenies of prime degree $p\in\{11,17,19,37,43,67,163\}$

Let $p\in\{11,17,19,37,43,67,163\}$ be a prime number. In [1], B. Mazur proves that there are only finite number of elliptic curves $E$ [over $\mathbb{Q}$] having an isogeny of degree $p$. Here is my ...
ZZP's user avatar
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Vector bundles on pro-etale topology over a field

Suppose $K$ is a finite extension of $\mathbb Q_p$. Consider the one-point adic space $X=\operatorname{Spa}K$, and let $C=\hat {\bar K}$, $G=\operatorname{Gal}(\bar K/K)$. I heard that the category of ...
Richard's user avatar
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1 answer
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Action of complex conjugation on etale cohomology

Let $X$ be a genus $g$ smooth projective curve, defined over $\mathbb{Q}$, and let $\overline{X}$ denote the base change of $X$ to $\overline{\mathbb{Q}}$. It is well known that $H^1_{\text{ét}}(\...
kindasorta's user avatar
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Lifting mod $p$ representations of arithmetic fundamental groups of a non-affine scheme over a finite field of characteristic $p$

Let $X$ be a geometrically irreducible scheme (not necessarily affine) over $\mathbb{F}_{p}$ and let $ \pi_{1}(X) $ be the arithmetic etale fundamental group of $ X $. Let $ \overline{\mathbb{F}}_{p} $...
Nobody's user avatar
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Absolute Bloch-Kato Cohomology

The étale cohomology $R\Gamma_{\mathrm{ét}}(X;\mathbb{Z}_p(n))$ of a scheme $X/K$ can be computed by a Hochschild-Serre spectral sequence with terms of the form $H^i(K;H^j(X_{\overline{K}};\mathbb{Z}...
David Corwin's user avatar
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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 ...
kindasorta's user avatar
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Local property of residual representations attached to elliptic curves over rational numbers

I found the following claim - without reference - in the (famous) book ''Modular Forms and Fermat’s Last Theorem'': Let $E$ be a semistable elliptic curve over $\mathbb{Q}$. Let $\Delta_E$ be the ...
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Unramified lisse $\overline{\mathbb{Q}}_{\ell}$-sheaves

Let $X$ be a connected noetherian scheme and $\ell$ a prime invertible on $X$. Let $D \subset X$ be a regular effective Cartier divisor (or more generally a normal crossings divisor, I suppose). Write ...
Hugo Zock's user avatar
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69 views

Global minimal discriminants of elliptic curves and Galois representations

Let $E$ be an elliptic curve over $\mathbb{Q}$. Let $\Delta$ be the global minimal discriminant. By A. Wiles et al. $E$ is modular so that there is a corresponding modular form $f$. I am wondering if ...
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Why locally algebraic characters of $\text{Gal}(\overline{\mathbb Q}/\mathbb Q)$ are associated to $A_0$ Grossencharacters/algebraic Hecke characters?

$\DeclareMathOperator\Res{Res}\DeclareMathOperator\Gal{Gal}$I am trying to understand lemma 3.1 of "Abelian Varieties over \mathbb Q and modular forms" of Ribet. ArXiv link Just so everyone ...
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93 views

Local units in a family of $S_4$-extensions

Let $a \in \mathbb{Z}$ and consider the polynomial $f(X)=X^4+aX+1$; we assume that $a$ is chosen such that $f$ is irreducible and that the discriminant $4^4-27a^4=-p$ for some prime $p$ (for example $...
Pol van Hoften's user avatar
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43 views

Absolute irreducibility implies free action on framed universal deformation ring

Let $\overline{\rho}: G\longrightarrow \text{GL}_n(\mathbb{F}_p)$ be a residual representation of the Galois group of a number field. Let $R_{\overline{\rho}}^{\square,\text{univ}}$ be the universal ...
kindasorta's user avatar
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Applications of Langlands for GLn explicit reciprocity laws other than elliptic curves

Is there any explicit application of Langlands conjecture for $\mathrm{GL}(n)$ for $n\ge 3$, to get some reciprocity laws for higher dimensional varieties or higher genus curves? I've never found such ...
Cloudifold's user avatar
4 votes
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111 views

The criterion for dimensional conjecture for universal Galois deformation rings

I’m writing to ask a question about Mazur’s dimensional conjecture in Lemma 7.5 of the paper [Galatius S, Venkatesh A. Derived Galois deformation rings. Advances in Mathematics. 2018 Mar 17;327:470-...
Nobody's user avatar
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1 answer
233 views

Existence of odd mod $p$ Galois representations whose image is $p'$-group

Let $K$ be a number field and let $G_K$ be the absolute Galois group of $K$. Let $p$ be an odd prime and $\mathbb{F}_p$ be a finite field of order $p$. Can we always find a continuous representation $\...
Nobody's user avatar
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Multiplicities of Galois representations in the semisimplification of the reduction of a Tate module

Let $C$ be a smooth proper curve, of genus $g$ over a number field $K$. Let $v$ be a prime of good reduction for $C$ above $p>2$, and let $T_pJ$ denote the $p$-adic Tate module of $J$, the Jacobian ...
kindasorta's user avatar
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1 vote
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A question related to Kirillov model

I am reading Jackson - The theory of admissible representations of $\operatorname{GL}(2, F)$ and am not able to understand the following map related to Kirillov model. This result appears on p. 54: I ...
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Prime to $p$ monodromy of local system on rigid variety

Suppose $F$ is a finite extension of $\mathbb Q_p$, and $X$ is a rigid variety over $F$. I saw in proposition 3.7 of Oswal, Shankar, Zhu, and Patel - A $p$-adic analogue of Borel's theorem: "Let $...
Richard's user avatar
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0 answers
99 views

On the irreducible submodules of adjoint representations $\text{ad}^{0}$

Let $k$ be a finite field of characteristic $p$. Let $H$ be a subgroup of $\rm{GL}_{n}(k)$ of order prime to $p$ where $n\geq2$. Assume that the representation $H\hookrightarrow \rm{GL}_{n}(k)$ is ...
stupid boy's user avatar
3 votes
1 answer
200 views

Discrepancy between $\dim H^2(G, \mathrm{ad}(\bar \rho))$ and the number of relations in a minimal presentation of the universal deformation ring $R$

$\DeclareMathOperator\GL{GL}\DeclareMathOperator\ad{ad}\DeclareMathOperator\gen{gen}$Let $p$ be a prime and $G$ be a profinite group such that the pro-$p$ completion of every open subgroup is ...
Nobody's user avatar
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7 votes
0 answers
371 views

Subgroups of ${\rm GL}_{2}(\mathbb{Z}/2^{n} \mathbb{Z})$

Assume that $n \geq 3$. Is there a subgroup $H \leq {\rm GL}_{2}(\mathbb{Z}/2^{n} \mathbb{Z})$ whose order is a power of $2$ so that $\bullet$ $\det : H \to (\mathbb{Z}/2^{n} \mathbb{Z})^{\times}$ is ...
Jeremy Rouse's user avatar
7 votes
1 answer
457 views

Symmetric power lift of modular forms

Let $f_1$ and $f_2$ be two cuspforms of weights $k_1$ and $k_2$ and nebentypus $\epsilon_1$ and $\epsilon_2$ respectively such that $f_1 \neq f_2 \otimes \chi$ for some Dirichlet character $\chi$ of ...
user15243's user avatar
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2 votes
0 answers
143 views

Prime splitting in the division field of an elliptic curve

Let $E/\mathbb{Q}$ be an elliptic curve with good reduction at two distinct primes $p, \ell$. Suppose the mod $\ell$ Galois representation associated to $E$ is surjective. Let $K=\mathbb{Q}(E[\ell])$ ...
Jeff H's user avatar
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17 votes
2 answers
981 views

Understanding absolute Galois group from its representations

Background. A major theme of modern number theory is to study the absolute Galois group $\text{Gal}(\overline{\mathbb Q} / \mathbb Q)$. Galois representation theory attempts to understand $\text{Gal}(\...
Uzu Lim's user avatar
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Ramification of mod $\ell$ representation of elliptic curves [closed]

Let $E$ be an elliptic curve over $\mathbb{Q}$, and let $p,\ell$ be two prime numbers. Consider the mod $\ell$ representation $$\rho:Gal(\mathbb{\overline{Q}}/\mathbb{Q})\to Aut(E[\ell])= GL(2,\ell).$$...
ZZP's user avatar
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1 vote
0 answers
99 views

Bounding dimension of $H^1(G_{\mathbb{Q}}, (V_pE)^{\otimes n})$

Let $E$ be an elliptic curve over $\mathbb{Q}$, let $p$ be a prime of good reduction, $T_pE$ is its $p$-adic Tate module, $V_pE = T_pE\otimes \mathbb{Q}_p$, and $(V_pE)^{\otimes n}$ its $n$'th tensor ...
kindasorta's user avatar
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0 votes
0 answers
83 views

Elliptic curves and images of decompositions group exceptional?

Given an elliptic curve $E$ and its mod $p$ Galois representation $\bar{\rho}_{E,p}$, I am wondering what are the possibilities for $\bar{\rho}_{E,p}(G_l)$, where $G_l:=$Gal($\overline{\mathbb{Q}_l}/\...
did's user avatar
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3 votes
0 answers
142 views

Taylor-Wiles systems for higher dimensional deformation rings

Let $R$ be a deformation ring and $M$ be a finitely generated $R$-module. A strategy for proving the theorems $R=T$ is to associate with $(R,M)$ a Taylor-Wiles system denoted $(R_{Q},M_{Q})$. Here I'm ...
Marsault Chabat's user avatar
2 votes
1 answer
290 views

Is the completed tensor product (over a complete dvr) of two reduced complete Noetherian local rings again reduced?

To be more specific, Let $\mathcal{O}$ be a finite extension of $\mathbb{Z}_{p}$. Let $A=\mathcal{O}[[X_{1},\ldots, X_{n}]]/\left( f_{1},\ldots,f_{r}\right) $ and $B=\mathcal{O}[[Y_{1},\ldots, Y_{m}]]/...
user521295's user avatar
1 vote
0 answers
127 views

Analytic properties of $L$-functions attached to a compatible system of $\ell$-adic Galois representations

Let $F$ and $E$ be number fields, $G_F$ be the absolute Galois group of $F$, and $S$ be a finite set of primes of $F$. For $\lambda$ a prime of $E$ we denote by $\ell$ its residual characteristic. We ...
LWW's user avatar
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2 votes
0 answers
140 views

Compatibility of system of $\ell$-adic representations associated to Voevodsky motives

Let $M$ be an object of Voevodsky's category $DM_{gm}(K,\mathbb{Q})$ for a number field $K$. For each prime number $\ell$, there is an $\ell$-adic realization $M_{\ell}$ in the bounded derived ...
David Corwin's user avatar
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3 votes
0 answers
261 views

Is the weight-monodromy conjecture known for unramified representations?

Let $X$ be a smooth proper variety over a number field $K$, $v$ a place of $K$ lying over a prime number $p \neq \ell$, and $V := H^n(X_{\overline{K}};\mathbb{Q}_{\ell})$. Suppose $V$ is unramified at ...
David Corwin's user avatar
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2 votes
0 answers
92 views

Local systems on $\mathbb P^1$ and on the formal punctured disc

Consider the projective curve $\mathbb P^1$ over a finite field $k$. Consider $\ell$-adic local systems $E$ on $\mathbb P^1\backslash \{0,\infty\}$ such that a) $E$ is tame at $\infty$ b) The ...
Alexander Braverman's user avatar
2 votes
1 answer
161 views

Norm 1 elements of an unramified quadratic extension of a local field

Let $E$ be an unramified quadratic extension of a local field $F$, with $p$ odd. Let $E^1$ denote the set of norm $1$ elements of $E$. What can be said about the following index: $$ {\rm 1.}\ \ \ \ [ ...
Ekta's user avatar
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4 votes
0 answers
160 views

A computation of nearby cycles

I'm currently reading P.Scholze's paper "THE LANGLANDS-KOTTWITZ APPROACH FOR THE MODULAR CURVE". In Lemma 7.7, he showed a (maybe simple) nearby cycle computation, which I can't follow. Now ...
Huang Chenxin's user avatar
3 votes
0 answers
101 views

List of techniques that have been used to prove topological properties of locus in the deformation ring or the Hecke algebra

My question is maybe going to be a bit vague. My apologies if so. The setting: Let $\overline{\rho}$ be a residual representation and $R$ be a deformation ring of $\overline{\rho}$. Let $\mathbb{T}$ ...
Marsault Chabat's user avatar
2 votes
1 answer
349 views

Families of Galois representations over disks

Edit on Nov. 20, 2023. This question is answered below in the case that $0<r_i<1$. And indeed it is shown in the answers to not be an interesting question in that case. So please take all $r_i=1$...
user avatar
2 votes
0 answers
183 views

Calculate $D_{\mathrm{cris}}(V)$ for a crystalline representation

$\newcommand{\cris}{\mathrm{cris}}$In my setting, $K/\mathbb Q_p$ is finite and unramified, and $V$ is a $2$-dimensional crystalline representation of $G_K$. Then we have $D_{\cris}(V)$, which is $2$-...
Richard's user avatar
  • 613
5 votes
1 answer
334 views

Conductor of determinant of a 2-dimensional Galois representation divides conductor of representation

I am studying Serre's paper "Sur les représentations modulaires de degré 2 de $\mathrm{Gal(\overline{\mathbb Q}/\mathbb Q)}$" and I'm stuck trying to prove that $N(\det\rho)$ divides $N(\rho)...
Marta Sánchez Pavón's user avatar
2 votes
1 answer
343 views

Irreducibility of Tate module (as a Galois representation) of elliptic curves with good reduction

This question is following the previous question. Definitions: Suppose $F$ is an unramified finite extension of $\mathbb Q_p$ and $E$ is an elliptic curve defined over $F$ with good reduction. Denote ...
Richard's user avatar
  • 613
3 votes
1 answer
273 views

Calculation of Frobenius on de Rham cohomology of elliptic curves with good reduction

I'm reading "An introduction to the theory of $p$-adic representations" by Laurent Berger. In the page 14 it says: Suppose $F$ is an unramified finite extension of $\mathbb Q_p$ and $E$ is ...
Richard's user avatar
  • 613
1 vote
0 answers
121 views

Is the Frobenius semisimple on the de-Rham cohomology?

Suppose $K$ is a unramified finite extension of $\mathbb Q_p$, and $X$ is a projective smooth curve defined over $K$. By $p$-adic Hodge theory we know $D_{cris}(H_{et}^i(X,\mathbb Q_p))=H_{dR}^i(X)$. ...
Richard's user avatar
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