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Search: MSC category 11S15 ( Ramification and extension theory )

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1. CMB 2007 (vol 50 pp. 594)

Laubie, François
Ramification des groupes abéliens d'automorphismes des corps $\mathbb F_q(\!(X)\!)$
Soit $q$ une puissance d'un nombre premier $p$. Dans cette note on \'etablit la g\'en\'eralisation suivante d'un th\'eor\`eme de Wintenberger : tout sous-groupe ab\'elien ferm\'e du groupe des $\mathbb F_q$-auto\-morphismes continus du corps des s\'eries formelles $\mathbb F_q(\!(X)\!)$ muni de sa filtration de ramification est un groupe filtr\'e isomorphe au groupe de Galois d'une extension ab\'elienne d'un corps local {\`a} corps r\'esiduel $\mathbb F_q$, filtr\'e par les groupes de ramification de l'extension en num\'erotation inf\'erieure.


2. CMB 2004 (vol 47 pp. 237)

Laubie, François
Ramification des séries formelles
Let $p$ be a prime number. Let $k$ be a finite field of characteristic $p$. The subset $X+X^2 k[[X]]$ of the ring $k[[X]]$ is a group under the substitution law $\circ $ sometimes called the Nottingham group of $k$; it is denoted by $\mathcal{R}_k$. The ramification of one series $\gamma\in\mathcal{R}_k$ is caracterized by its lower ramification numbers: $i_m(\gamma)=\ord_X \bigl(\gamma^{p^m} (X)/X - 1\bigr)$, as well as its upper ramification numbers: $$ u_m (\gamma) = i_0 (\gamma) + \frac{i_1 (\gamma) - i_0(\gamma)}{p} + \cdots + \frac{i_m (\gamma) - i_{m-1} (\gamma)}{p^m} , \quad (m \in \mathbb{N}). $$ By Sen's theorem, the $u_m(\gamma)$ are integers. In this paper, we determine the sequences of integers $(u_m)$ for which there exists $\gamma\in\mathcal{R}_k$ such that $u_m(\gamma)=u_m$ for all integer $m \geq 0$.

Keywords:ramification, Nottingham group
Categories:11S15, 20E18

3. CMB 2002 (vol 45 pp. 168)

Byott, Nigel P.; Elder, G. Griffith
Biquadratic Extensions with One Break
We explicitly describe, in terms of indecomposable $\mathbb{Z}_2 [G]$-modules, the Galois module structure of ideals in totally ramified biquadratic extensions of local number fields with only one break in their ramification filtration. This paper completes work begun in [Elder: Canad. J.~Math. (5) {\bf 50}(1998), 1007--1047].

Categories:11S15, 20C11

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