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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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-...
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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 $\...
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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 ...
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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 $...
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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 ...
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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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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 ...
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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 ...
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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 $K=\mathbb{Q}(E[\ell])$ ...
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17 votes
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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}(\...
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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 $$\rho:Gal(\mathbb{\overline{Q}}/\mathbb{Q})\to Aut(E[\ell])= GL(2,\ell).$$...
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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 ...
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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}/\...
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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 ...
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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}]]/...
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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 ...
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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 ...
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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 ...
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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
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1 answer
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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.}\ \ \ \ [ ...
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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 ...
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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
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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 $r_i=1$...
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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 $2$-...
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1 answer
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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)...
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1 answer
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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 ...
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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 ...
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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 $D_{cris}(H_{et}^i(X,\mathbb Q_p))=H_{dR}^i(X)$. ...
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Is there a bound on the number of $p$-adic semisimple representations?

Faltings proved the following: Fix integers $w, d \geqslant 0$, and fix a number field $K$ and a finite set $S$ of primes of $\mathcal{O}_K$. There are, up to conjugation, only finitely many ...
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Kernel of restriction map in Galois cohomology

Let $S$ be the algebraic group $SL_2/\mathbb{Q}_p$ with a $G=G_{\mathbb{Q}}$ action, (acts by conjugation with a representation $\rho: G\longrightarrow GL_2$.) Let $G_p$ be the decomposition group at ...
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Description of $\operatorname{Gal}(K(E[n])/K)$ as a subgroup of $\operatorname{GL}_2(\mathbb{Z}/n\mathbb{Z})$ for a CM elliptic curve $E$

I am looking for a specific description of the Galois groups $\operatorname{Gal}(K(E[n])/K)$ as a subgroup of $\operatorname{GL}_2(\mathbb{Z}/n\mathbb{Z})$ for an elliptic curve $E$ with complex ...
Anish Ray's user avatar
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1 answer
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Two different local Langlands parameters for quadratic extension

Let $E/F$ be a quadratic extension of nonarchimedean local field, and let $G$ be a reductive group over $E$, and $G(E)$ the $E$-points of $G$. (For my question, one may just assume $G=\operatorname{GL}...
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On odd Fourier coefficients of (elliptic) modular forms

Let $E/\mathbb{Q}$ be any elliptic curve with no non-trivial $2$ torsion, and $\rho_E:\mathrm{Gal}(\overline{\mathbb{Q}}/\mathbb{Q})\to \mathrm{Aut}(E[2])=\mathrm{GL}_2(\mathbb{Z}/2\mathbb{Z})\cong ...
dragoboy's user avatar
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Galois weight of a Serre twisted pure Galois representation

Let $V$ be a pure, 2-dimensional weight $k$ $p$-adic global Galois representation for $G$, the absolute Galois group of the rationals. Let $c\in Z^1(G,\text{SL}_2)$ be a cocycle and consider the Serre ...
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Galois extension generated by 2-torsion points

Let $E/\mathbb{Q}$ be an elliptic curve such that $E(\mathbb{Q})[2]$ is trivial. Hence image of the well known Galois representation $$\rho:\mathrm{Gal}(\overline{\mathbb{Q}}/\mathbb{Q})\to \mathrm{...
dragoboy's user avatar
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A variant of the inverse Galois problem

In Theorem I of Construction of maximal unramified p-extensions with prescribed Galois groups, it's proved that for any prime $p$ and any given finite $p$-group $G$, there exists a number field $F$ ...
stupid boy's user avatar
3 votes
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Reference showing no proper subgroups of p-adic orthogonal groups surject onto mod p orthogonal groups

I am looking for a reference for the following statement: Let $O$ be an orthogonal group associated to a nondegenerate quadratic form of rank $r$ over the p-adic integers $\mathbb Z_p$. Suppose $r$ is ...
Aaron Landesman's user avatar
2 votes
1 answer
237 views

$\pi$-adic Galois representations of attached to newforms at $p \nmid N$ are crystalline

Is [Scholl, Motives for modular forms, Theorem 1.2.4 (ii)] proven for any $p$ independent of the weight? Concretely, let $f$ be a normalized eigenform of weight $w$. Let $p$ be a prime not dividing ...
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5 votes
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183 views

Image of $\gamma-1$ on etale $(\varphi,\Gamma)$-modules

Let $p\geq 3$ be a prime, $D$ be an etale $(\varphi,\Gamma)$-modules over the classical period ring $A_{\mathbb{Q}_p}=\mathbb{Z}_p[\![T]\!][1/T]^{\widehat{\phantom{xx}}}_p$ and $\gamma$ be a ...
Yijun Yuan's user avatar
2 votes
1 answer
336 views

Galois cohomology of Tate modules

Let $E,E'$ be a pair of elliptic curves defined over $\mathbb{Z}$. Let $T_p[E], T_p[E']$ be their associated ($p$-adic) Tate modules. These are Galois representations for the absolute Galois group of $...
kindasorta's user avatar
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3 votes
1 answer
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Local Tate duality for F-vector space

A version of local Tate duality stated: Let $K$ be a finite extension of $\mathbb Q_p$, $A$ be a finite $G_K=Gal(\overline K/K)$ module. Then for $0\le i\le 2$, the cup product induces a perfect ...
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Inertia group representation from $p^{n}$-torsion of ordinary elliptic curve

Let $K$ be a complete local field. Suppose that $K$ is an unramified extension of $\mathbb{Q}_{p}$ and let $E$ be an elliptic curve over $K$ with good ordinary reduction. Let $G_{K}=\text{Gal}(\...
David Hubbard's user avatar
36 votes
1 answer
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What is the smallest group not known to be a Galois group over $\mathbb{Q}$?

$\DeclareMathOperator\SL{SL}\DeclareMathOperator\PSL{PSL}$What is the smallest group not known to be a Galois group over $\mathbb{Q}$? Variants have been asked here before (e.g. Which small finite ...
Joachim König's user avatar
2 votes
1 answer
203 views

Elliptic curve with CM and image of Galois representation in normalizer of nonsplit Cartan

I am trying to understand the following. Let $E/\mathbb{Q}$ be an elliptic curve with complex multiplication given by the ring of integers $\mathcal{O}_K$. We are given a fixed rational prime $p$ ...
did's user avatar
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1 vote
1 answer
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About simple motives

I'm reading through Jannsen's paper Motives, numerical equivalence, and semi-simplicity and I'd like to pose two questions. Suppose all motives are $F$-linear, for some characteristic zero field $F$, ...
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3 votes
0 answers
180 views

Representability of $\operatorname{Hom}(G_{\mathbb{Q}}, \operatorname{GL}_2)$

Let $G_{\mathbb{Q}}$ be the absolute Galois group of the rationals, and let $F: \mathrm{Aff}/\textbf{Q}_p\longrightarrow \mathrm{Sets}$ be the functor which associates to every affine $\mathbb{Q}_p$ ...
kindasorta's user avatar
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5 votes
1 answer
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Understand the $p$-adic local Langlands correspondence with examples

Let $\rho:G_{\mathbb{Q}}\rightarrow \mathbf{Gl}_{n}(\mathbb{Q}_{p})$. I would like to understand in depth why the local Langlands correspondence for $\rho_{|\mathbb{Q}_{p}}$ must consider $p$-adic ...
Marsault Chabat's user avatar
4 votes
0 answers
200 views

Effect of the surjectivity of Galois representation

Let $K$ be any arbitrary number field and $E$ be any elliptic curve over it. For any integer $m,$ consider the well-known Galois representation $$\rho:\text{Gal}(\overline{K}/K)\to \text{GL}_2(\mathbb{...
dragoboy's user avatar
  • 521
4 votes
1 answer
239 views

Double centralizer theorem for ($\ell$-adic) Lie algebras

$\DeclareMathOperator\GL{GL}$I'm reading the proof of Serre's open image theorem from his book "Abelian $\ell$-adic representations and elliptic curves". This is chapter IV Section 2.2 of ...
Alireza Shavali's user avatar

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