Kähler differentials, almost mathematics and deeply ramified fields

Abstract: This article discusses ramification and the structure of relative K\"ahler differentials of extensions of valued fields. We begin by surveying the theory developed in recent work with Franz-Viktor Kuhlmann and Anna Rzepka constructing the relative K\"ahler differentials of extensions of valuation rings in Artin-Schreier and Kummer extensions. We then show how this theory is applied to give a simple proof of Gabber and Ramero's characterization of deeply ramified fields. Section 4 develops the basics of almost mathematics, and should be accessible to a broad audience. Section 5 gives a simple and self contained proof of Gabber and Ramero's characterization of when the extension of a rank 1 valuation of a field to its separable closure is weakly \'etale. In the final section, we consider the equivalent conditions characterizing deeply ramified fields, as they are defined by Coates and Greenberg, and show that they are the same as the conditions of Gabber Ramero for local fields.