Degré des diviseurs sur les families de courbes de P3.
Nous nous intéressons aux composantes irréductibles des espaces de modules de G-revêtements et à leurs corps de définition. Nos résultats permettent de construire, quel que soit le groupe fini, de telles composantes définies sur . Notre méthode laisse de plus une grande latitude quant au type de ramification des revêtements. Ces composantes sont obtenues par déformation de certains revêtements du bord des espaces de modules. Enfin, ces composantes sont aussi compatibles dans une tour d’espaces...
The main purpose of this paper is twofold. We first analyze in detail the meaningful geometric aspect of the method introduced in [12], concerning families of irreducible, nodal curves on a smooth, projective threefold X. This analysis gives some geometric interpretations not investigated in [12] and highlights several interesting connections with families of other singular geometric objects related to X and to other varieties. Then we use this method to study analogous problems for families of...
In this paper we consider questions of the following type. Let be a base field and be a field extension. Given a geometric object over a field (e.g. a smooth curve of genus ), what is the least transcendence degree of a field of definition of over the base field ? In other words, how many independent parameters are needed to define ? To study these questions we introduce a notion of essential dimension for an algebraic stack. Using the resulting theory, we give a complete answer to...
Let be the moduli space of -pointed Riemann surfaces of genus . Denote by the Deligne-Mumford compactification of . In the present paper, we calculate the orbifold and the ordinary Euler characteristic of for any and such that .