Let (R, m) be a local GCD domain. R is called a U2 ring if there is an element u ∈ m − m 2 such that R/(u) is a valuation domain and Ru is a Bézout domain. In this case u is called a normal element of R. In this paper we prove that if R is a U2 ring, then R and R[x] are coherent; moreover, if R has a normal element u and dim(R/(u)) = 1, then every finitely generated projective module over R[X] is free. Keywords: weak global dimension, local domain, projective, free MSC(2000): 13C10, 13D05, 13G05
IntroductionLet R be a commutative ring and R[X 1 , . . . , X n ] be the polynomial ring over R., then M is finitely generated projective over R. It is clear that a free R[X 1 , . . . , X n ]-module is extended from R. By Kaplansky theorem [1] , if a finitely generated projective R[X 1 , . . . , X n ]-module H is extended from R and R is local, then H is free. The problem whether finitely generated projective R[X 1 , . . . , X n ]-modules are extended from R originates from Serre conjecture [2] . In 1955, Serre guessed that, for a field K, each finitely generated projective K[X 1 , . . . , X n ]-module is free. Serre Conjecture was solved independently by Quillen and Suslin in 1976. The problem has an extension to Noetherian regular rings and it is called Bass-Quillen Conjecture (for short, BQ), that is, BQ: Let R be a regular Noetherian local ring. Then every finitely generated projective R[X 1 , . . . , X n ]-module is extended from R.By the Quillen's patching theorem [3] , a finitely presented R[X 1 , . . . , X n ]-module H is extended from R if and only if H m is extended from R m for any maximal ideal m of R. Thus Bass-Quillen Conjecture is equivalent to the following form:BQ d : Let R be a d-dimensional regular Noetherian local ring. Then every finitely generated projective R[X 1 , . . . , X n ]-module is free, where d = gl. dim(R) = dim(R).In the case d 2 Bass-Quillen Conjecture is true (see [4]). For the case d = 3, Rao proved in [5] that Bass-Quillen Conjecture is true if the characteristic is not equal to 2 or 3. Many