Almost purity and overconvergent Witt vectors

Abstract

In a previous paper, we stated a general almost purity theorem in the style of Faltings: if R is a ring for which the Frobenius maps on finite p-typical Witt vectors over R are surjective, then the integral closure of R in a finite \'etale extension of R[p^{-1}] is "almost" finite \'etale over R. Here, we use almost purity to lift the finite \'etale extension of R[p^{-1}] to a finite \'etale extension of rings of overconvergent Witt vectors. The point is that no hypothesis of p-adic completeness is needed; this result thus points towards potential global analogues of p-adic Hodge theory. As an illustration, we construct (phi, Gamma)-modules associated to Artin Motives over Q. The (phi, Gamma)-modules we construct are defined over a base ring which seems well-suited to generalization to a more global setting; we plan to pursue such generalizations in later work.