Nous développons une nouvelle stratégie pour comprendre la nature des obstructions aux déformations d’une représentation galoisienne globale $\overline{\rho }$ réductible, impaire de dimension 2. Ces obstructions s’interprètent en termes de groupe de Šafarevič. D’après [BöMé], elles sont reliées à des conjecture arithmétiques classiques (Conjecture de Vandiver, conjecture de Greenberg). Dans cet article, nous introduisons un autre groupe de Šafarevič associé au corps $L$ fixe par $ker\overline{\rho }$. Nous comparons les deux groupes...

For a finite group G let 𝒦₂(G) denote the set of normal number fields (within ℂ) with Galois group G which are 2-ramified, that is, unramified outside {2,∞}. We describe the 2-groups G for which 𝒦₂(G) ≠ ∅, and determine the fields in 𝒦₂(G) for certain distinguished 2-groups G appearing (dihedral, semidihedral, modular and semimodular groups). Our approach is based on Fröhlich's theory of central field extensions, and makes use of ring class field constructions (complex multiplication).

We introduce a notion of “Galois closure” for extensions of rings. We show that the notion agrees with the usual notion of Galois closure in the case of an $S_n$ degree $n$ extension of fields. Moreover, we prove a number of properties of this construction; for example, we show that it is functorial and respects base change. We also investigate the behavior of this Galois closure construction for various natural classes of ring extensions.

We show that the discriminant of the generalized Laguerre polynomial $L_n^{\left(\alpha \right)}\left(x\right)$ is a non-zero square for some integer pair $(n,\alpha )$, with $n\ge 1$, if and only if $(n,\alpha )$ belongs to one of $30$ explicitly given infinite sets of pairs or to an additional finite set of pairs. As a consequence, we obtain new information on when the Galois group of $L_n^{\left(\alpha \right)}\left(x\right)$ over $\mathbb{Q}$ is the alternating group $A_n$. For example, we establish that for all but finitely many positive integers $n\equiv 2(mod4)$, the only $\alpha$ for which the Galois group of $L_n^{\left(\alpha \right)}\left(x\right)$ over $\mathbb{Q}$ is $A_n$ is $\alpha =n$.

Let $p\ge 3$ be a prime, let $n\>m\ge 1$. Let ${\pi }_n$ be the norm of ${\zeta }_{p^n}-1$ under $C_{p-1}$, so that ${\mathbb{Z}}_{\left(p\right)}\left[{\pi }_n\right]|{\mathbb{Z}}_{\left(p\right)}$ is a purely ramified extension of discrete valuation rings of degree $p^{n-1}$. The minimal polynomial of ${\pi }_n$ over $\mathbb{Q}\left({\pi }_m\right)$ is an Eisenstein polynomial; we give lower bounds for its coefficient valuations at ${\pi }_m$. The function field analogue, as introduced by Carlitz and Hayes, is studied as well.

Let $\mathbb{Q}\left(\alpha \right)$ be an algebraic number field given by the minimal polynomial $f$ of $\alpha$. We want to determine all subfields $\mathbb{Q}\left(\beta \right)\subset \mathbb{Q}\left(\alpha \right)$ of given degree. It is convenient to describe each subfield by a pair $(g,h)\in \mathbb{Z}\left[t\right]\times \mathbb{Q}\left[t\right]$ such that $g$ is the minimal polynomial of $\beta =h\left(\alpha \right)$. There is a bijection between the block systems of the Galois group of $f$ and the subfields of $\mathbb{Q}\left(\alpha \right)$. These block systems are computed using cyclic subgroups of the Galois group which we get from the Dedekind criterion. When a block system is known we compute the corresponding...

Let α, β and γ be algebraic numbers of respective degrees a, b and c over ℚ such that α + β + γ = 0. We prove that there exist algebraic numbers α₁, β₁ and γ₁ of the same respective degrees a, b and c over ℚ such that α₁ β₁ γ₁ = 1. This proves a previously formulated conjecture. We also investigate the problem of describing the set of triplets (a,b,c) ∈ ℕ³ for which there exist finite field extensions K/k and L/k (of a fixed field k) of degrees a and b, respectively, such that the degree of the...