Tame fields, Graded Rings and Finite Complete Sequences of Key Polynomials

In this paper, we link the concept of tame field to the study of graded rings and complete sequences of key polynomials. We review some definitions and results on the theory of key polynomials and Mac Lane-Vaqui\'e chains which enable us to prove that a valued field $(K,v)$ is tame if and only if the Frobenius endomorphism on the associated graded ring is surjective and every simple algebraic extension of K admits a finite complete sequence of key polynomials. We present a criterion for $(K,v)$ to be henselian and defectless in terms of finite complete sequences of key polynomials. We also make some considerations on purely inertial and purely ramified extensions and their relations to properties of the associated graded ring and the module of K\"ahler differentials.