Néron models and tame ramification
Given a scheme in characteristic p together with a lifting modulo p2, we construct a functor from a category of suitably nilpotent modules with connection to the category of Higgs modules. We use this functor to generalize the decomposition theorem of Deligne-Illusie to the case of de Rham cohomology with coefficients.
Let be a field of odd characteristic , let be an irreducible separable polynomial of degree with big Galois group (the symmetric group or the alternating group). Let be the hyperelliptic curve and its jacobian. We prove that does not have nontrivial endomorphisms over an algebraic closure of if either or .
We give an infinite family of curves of genus 2 whose Jacobians have non-trivial members of the Tate-Shafarevich group for descent via Richelot isogeny. We prove this by performing a descent via Richelot isogeny and a complete 2-descent on the isogenous Jacobian. We also give an explicit model of an associated family of surfaces which violate the Hasse principle.