-crystals on schemes with constant log structure
Soient un nombre premier et un corps -adique à corps résiduel parfait (par exemple une extension finie de ) dont l’indice de ramification absolue est noté . Afin d’étudier les « représentations semi-stables de -torsion » de , Breuil a défini pour tout entier positif plusieurs catégories de -modules filtrés de torsion. Dans cet article, nous décrivons la structure de ces catégories dans le cas général (seul le cas avait été étudié de façon systématique jusqu’à présent).
A polynomial f in the set {Xⁿ+Yⁿ, Xⁿ +Yⁿ-Zⁿ, Xⁿ +Yⁿ+Zⁿ, Xⁿ +Yⁿ-1} lends itself to an elementary proof of the following theorem: if the coordinate ring over ℚ of f is factorial, then n is one or two. We give a list of problems suggested by this result.
We describe a relation between the invariants of ordered points in projective -space and of points contained in a union of two linear subspaces. This yields an attaching map for GIT quotients parameterizing point configurations in these spaces, and we show that it respects the Segre product of the natural GIT polarizations. Associated to a configuration supported on a rational normal curve is a cyclic cover, and we show that if the branch points are weighted by the GIT linearization and the rational...
Given any global field of characteristic , we construct a Châtelet surface over that fails to satisfy the Hasse principle. This failure is due to a Brauer-Manin obstruction. This construction extends a result of Poonen to characteristic , thereby showing that the étale-Brauer obstruction is insufficient to explain all failures of the Hasse principle over a global field of any characteristic.