Reduction and lifting of special metacyclic covers
We show explicit forms of the Bertini-Noether reduction theorem and of the Hilbert irreducibility theorem. Our approach recasts in a polynomial context the geometric Grothendieck good reduction criterion and the congruence approach to HIT for covers of the line. A notion of “bad primes” of a polynomial P ∈ ℚ[T,Y] irreducible over ℚ̅ is introduced, which plays a central and unifying role. For such a polynomial P, we deduce a new bound for the least integer t₀ ≥ 0 such that P(t₀,Y) is irreducible...
Let and be two compact strongly pseudoconvex CR manifolds of dimension which bound complex varieties and with only isolated normal singularities in and respectively. Let and be the singular sets of and respectively and is nonempty. If and the cardinality of is less than 2 times the cardinality of , then we prove that any non-constant CR morphism from to is necessarily a CR biholomorphism. On the other hand, let be a compact strongly pseudoconvex CR manifold of...
Nous développons une version de la théorie d’indice d’Atiyah pour les faisceaux cohérents sur les variétés algébriques lisses et l’utilisons pour attaquer certaines questions de J. Kollár.Soit une variété complexe compacte projective algébrique lisse et connexe. Nous prouvons que si est un diviseur nef et gros, tel que la restriction de à la fibre générale d’une application de Shafarevich est effective, est effectif.Soit une variété kählérienne compacte telle qu’il existe une classe...