Torsion points on elliptic curves and galois module structure.
Let be a Henselian discrete valuation ring with field of fractions . If is a smooth variety over and a torus over , then we consider -torsors under . If is a model of then, using a result of Brahm, we show that -torsors under extend to -torsors under a Néron model of if is split by a tamely ramified extension of . It follows that the evaluation map associated to such a torsor factors through reduction to the special fibre. In this way we can use the geometry of the special...
We extend Ogus’notion of -crystal and -span to the context of Berthelot’s crystals of level and we study the generalization of Ogus’theorem on the equivalence between -crystals and -spans of width less than .