LAUSR

Abstract Let v be a Krull valuation of a field K with valuation ring R v and K 1 , K 2 be finite separable extensions of K which are linearly disjoint over K. Assume that the integral closure of R v in the composite field K 1 K 2 is a free R v -module. For a given pair of prolongations v 1 , v 2 of v to K 1 , K 2 respectively, it is shown that there exists a unique prolongation w of v to K 1 K 2 which extends both v 1 , v 2 . Moreover with S i as the integral closure of R v in K i , if the ring S 1 S 2 is integrally closed and the residue field of v is perfect, then f ( w / v ) = f ( v 1 / v ) f ( v 2 / v ) , where f ( v ′ / v ) stands for the degree of the residue field of a prolongation v ′ of v over the residue field of v. As an application, it is deduced that if K 1 , K 2 are algebraic number fields which are linearly disjoint over K = K 1 ∩ K 2 , then the number of prime ideals of the ring A K 1 K 2 of algebraic integers of K 1 K 2 lying over a given prime ideal ℘ of A K equals the product of the numbers of prime ideals of A K i lying over ℘ for i = 1 , 2 .