We study equivariant deformations of singular curves with an action of a finite flat group scheme, using a simplified version of Illusie's equivariant cotangent complex. We apply these methods in a special case which is relevant for the study of the stable reduction of three point covers.
Continuing our work on the fundamental groups of conic-line arrangements (Amram et al., 2003), we obtain presentations of fundamental groups of the complements of three families of quadric arrangements in P2. The first arrangement is a union of n conics, which are tangent to each other at two common points. The second arrangement is composed of n quadrics which are tangent to each other at one common point. The third arrangement is composed of n quadrics, n-1 of them are tangent to the n-th one...
Our main result combines three topics: it contains a Grunwald-Wang type conclusion, a version of Hilbert’s irreducibility theorem and a -adic form à la Harbater, but with good reduction, of the Regular Inverse Galois Problem. As a consequence we obtain a statement that questions the RIGP over . The general strategy is to study and exploit the good reduction of certain twisted models of the covers and of the associated moduli spaces.
This is the material which I presented at the 60th birthday conference of my good friend José Luis Vicente in Seville in September 2001. It is based on the nine lectures, now called sections, which were given by me at Purdue in Spring 1997. This should provide a good calculational background for the Galois theory of vectorial ( = additive) polynomials and their iterates.
À tout dessin d’enfant est associé un revêtement ramifié de la droite projective complexe , non ramifié en dehors de 0, 1 et l’infini. Cet article a pour but de décrire la structure algébrique de l’image réciproque de la droite projective réelle par ce revêtement, en termes de la combinatoire du dessin d’enfant. Sont rappelées en annexe les propriétés de la restriction de Weil et des dessins d’enfants utilisées.
Dans cet article nous nous intéressons aux propriétés des composantes irréductibles associées à la géométrie réelle d’un dessin d’enfant. Plus précisément, nous étudions les composantes irréductibles de la courbe dont l’ensemble des points réels est l’image réciproque de par une fonction de Belyi d’un dessin d’enfant.
We describe the rigid geometry of the first layer in the tower of coverings of the -adic upper half plane constructed by Drinfeld. Using our results, we describe the stable fiber at p of certain Shimura curves.
A simple calculation of the Hasse-Witt matrix is used to give examples of curves which are Kummer coverings of the projective line and which have easily determined p-rank. A family of curve carrying non-classical vector bundles of rank 2 is also given.
Let E be an elliptic curve over a field K of characteristic not equal to 2 and let N > 1 be an integer prime to char(K). The purpose of this paper is to construct the (two-dimensional) Hurwitz moduli space H (E/K,N,2) which classifies genus 2 covers of E of degree N and to show that it is closely related to the modular curve X(N) which parametrizes elliptic curves with level-N-structure. More precisely, we introduce the notion of a normalized genus 2 cover of E/K and show that the corresponding...