The main purpose of this paper is to establish a result giving the number of intermediary rings between R and S when (R, S) is a normal pair of rings and to provide an algorithm to compute this number. c 2007 Published by Elsevier B.V. MSC: Primary: 13B02; secondary: 13C15; 13A17; 13A18; 13B25; 13E05
IntroductionThis paper is a sequel to [4], we adopt the conventions that each ring considered is commutative, with unit and an inclusion (extension) of rings signifies that the smaller ring is a subring of the larger and possesses the same multiplicative identity. Throughout this paper, Spec(R) denotes the set of prime ideals of an integral domain R, and Max(R) denotes the set of its maximal ideals. The height of a prime ideal P of R denoted by ht P, is defined to be the supremum of the lengths n of chains of prime ideals of R, P 0 ⊂ P 1 ⊂ · · · ⊂ P n = P. The Krull dimension of R, dim(R), is defined to be the supremum of such heights for P ∈ Max(R). If P ⊆ Q are two prime ideals of R, then [P, Q] will denote the set of prime ideals Q of R such that P ⊆ Q ⊆ Q. For any set X , |X | denotes the cardinality of this set. Let R ⊆ S be an extension of integral domains. The set of subrings of S that contain R is called the set of intermediary rings in the ring extension R ⊆ S. We let [R, S] denote this set. If K is the quotient field of R, then an intermediate ring in the extension R ⊆ K is called an overring of R. If each overring of R is integrally closed in K , then R is called a Prüfer domain (cf. [9]).Recall that a pair of rings (R, S) is called a normal pair provided that R ⊆ S and each T ∈ [R, S] is integrally closed in S. These pairs were first defined and studied by Davis [6]. For example, if R is a Prüfer domain with quotient field K , then (R, K ) is a normal pair. Davis demonstrated that if R is a local ring, then (R, S) is a normal pair if and