We discuss new decidability and undecidability results for mixed characteristic henselian fields, whose proof goes via reduction to positive characteristic. The reduction uses extensively the theory of perfectoid fields and also the earlier Krasner-Kazhdan-Deligne principle. Our main results will be:
(1) A relative decidability theorem for perfectoid fields. Using this, we obtain decidability of certain tame fields of mixed characteristic.
(2) An undecidability result for the asymptotic theory of all finite extensions of ℚ_p (fixed p) with cross-section.
We will also discuss a tentative step towards understanding the underlying model theory of arithmetic phenomena in this area, by presenting a model-theoretic way of seeing the Fontaine-Wintenberger theorem
- Séminaire Géométrie et théorie des modèles