Valuations in algebraic field extensions

Let K → L be an algebraic field extension and ν a valuation of K. The purpose of this paper is to describe the totality of extensions {ν′} of ν to L using a refined version of MacLane’s key polynomials. In the basic case when L is a finite separable extension and rk ν = 1, we give an explicit description of the limit key polynomials (which can be viewed as a generalization of the Artin–Schreier polynomials). We also give a realistic upper bound on the order type of the set of key polynomials. Namely, we show that if char K = 0 then the set of key polynomials has order type at most N, while in the case char K = p > 0 this order type is bounded above by logp n + 1 ω, where n = [L : K]. Our results provide a new point of view of the the well known formula Ps j=1 ejfjdj = n and the notion of defect.Ministerio de Educación y CienciaFondo Europeo de Desarrollo Regiona