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Relative Fp-Projective Modules Communications in Algebra®, 33: 1587–1602, 2005 Copyright © Taylor & Francis, Inc. ISSN: 0092-7872 print/1532-4125 online DOI: 10.1081/AGB-200061047 RELATIVE FP-PROJECTIVE MODULES# Lixin Mao Department of Basic Courses, Nanjing Institute of Technology, and Department of Mathematics, Nanjing University, Nanjing, P.R. China Nanqing Ding Department of Mathematics, Nanjing University, Nanjing, P.R. China Let R be a ring and M a right R-module. M is called n-FP-projective if Ext1M N = 0 for any right R-module N of FP-injective dimension ≤n, where n is a = nonnegative integer or n . RM is defined as supn M is n-FP-projective and =− 1 = RM 1ifExt M N 0 for some FP-injective right R-module N. The right -dimension r-dimR of R is defined to be the least nonnegative integer n such that ≥ = RM n implies RM for any right R-module M. If no such n exists, set r-dimR =. The aim of this paper is to investigate n-FP-projective modules and the -dimension of rings. Key Words: Cotorsion theory; -Dimension; FP-injective dimension; n-FP-Projective module. Mathematics Subject Classification: Primary 16E10; Secondary 16D40. 1. NOTATION In this section, we shall recall some known notions and definitions which we need in the later sections. Throughout this paper, R is an associative ring with identity and all modules are unitary R-modules. We write MR (RM) to indicate a right (left) R-module. If ···→ → → → = P1 P0 M 0 is a projective resolution of an R-module M, let K0 = → = → ≥ ≥ M K1 kerP0 M Ki kerPi−1 Pi−2 for i 2. The nth kernel Kn n 0 is called the nth syzygy of M. As usual, wDR stands for the weak global dimension of R. pdM, idM and fdM denote the projective, injective and flat dimensions n n of M respectively. HomM N Ext M N) means HomRM N ExtRM N) for ≥ R an integer n 1, and similarly TornM N denotes Torn M N unless otherwise specified. For other concepts and notations, we refer the reader to Anderson and Fuller (1974), Enochs and Jenda (2000), Wisbauer (1991), and Xu (1996). Let R be a ring and M a right R-module. Received January 14, 2004; Revised September 9, 2004 #Communicated by A. Facchini. Address correspondence to Nanqing Ding, Department of Mathematics, Nanjing University, Nanjing 210093, P.R. China; E-mail: [email protected] 1587 1588 MAO AND DING M is called FP-injective (or absolutely pure) (Madox, 1967 and Stenström, 1970) if Ext1N M = 0 for all finitely presented right R-modules N. Following Stenström (1970), the FP-injective dimension of M, denoted by FP-idM, is defined to be the smallest integer n ≥ 0 such that Extn+1F M = 0 for every finitely presented right R-module F (if no such n exists, set FP-idM =), and rFP- dimR is defined as supFP-idM M is a right R-module}. In Mao and Ding (2005), the FP-projective dimension fpdM of M is defined to be the smallest integer n ≥ 0 such that Extn+1M N = 0 for any FP-injective right R-module N. If no such n exists, set fpdM =. The right FP-projective dimension rfpDR of a ring R is defined as sup{fpdM M is a finitely generated right R-module}. M is called FP-projective if fpdM = 0. Clearly, fpdM measures how far away a right R-module M is from being FP-projective. Enochs (1976) proved that a finitely generated right R-module M is finitely presented if and only if Ext1M N = 0 for any FP-injective right R-module N, and so a finitely generated FP-projective right R-module is finitely presented. It follows that rfpDR measures how far away a ring R is from being right Noetherian. A ring R is called right coherent if every finitely generated right ideal of R is finitely presented. A pair of classes of right R-modules is called a cotorsion theory (Enochs and Jenda, 2000) if ⊥ = and ⊥ = , where ⊥ = C Ext1F C = 0 for all F ∈ , and ⊥ = F Ext1F C = 0 for all C ∈ . Let be a class of right R-modules and M a right R-module. A homo- morphism M→ F with F ∈ is called a -preenvelope of M (Enochs and Jenda, 2000) if for any homomorphism f: M → F with F ∈ , there is a homomorphism gF→ F such that g = f. Moreover, if the only such g are automorphisms of F when F = F and f = , the -preenvelope is called a -envelope of M.A-envelope M→ F is said to have the unique mapping property (Ding, 1996) if for any homomorphism f: M → F with F ∈ , there is a unique homomorphism gF→ F such that g = f. Following Enochs and Jenda (2000), Definition 7.1.6, a monomorphism M→ C with C ∈ is said to be a special -preenvelope of M if coker ∈ ⊥. Dually we have the definitions of a (special) -precover and a -cover (with the unique mapping property). Special - preenvelopes (respectively special -precovers) are obviously -preenvelopes (resp. -precovers). 2. INTRODUCTION The FP-projective dimension of modules was introduced in Mao and Ding (2005) to measure how far away a module is from being FP-projective. In this paper, we approach FP-projective modules from another point of view and introduce the concepts of n-FP-projective modules and -dimensions of modules and rings. A right R-module M is called n-FP-projective if Ext1M N = 0 for all right R-modules N of FP-injective dimension ≤n, where n is a nonnegative integer or n =. We define =− 1 = RM = supnMis n-FP-projective} and RM 1ifExtM N 0 for some FP-injective right R-module N. The right -dimension r · -dimR of a ring R is ≥ = defined to be the least nonnegative integer n such that RM n implies RM for any right R-module M. If no such n exists, set r · -dimR =. The purpose of this paper is to investigate these new notions. RELATIVE FP-PROJECTIVE MODULES 1589 Let R be a right coherent ring and n a fixed nonnegative integer. In Section 3, we prove that ( n n is a cotorsion theory, moreover, every right R-module has a special n-preenvelope, and every right R-module has a special n-precover, where n ( n) denotes the class of all right modules of FP-injective dimension ≤n (all n-FP-projective right R-modules) (see Theorem 3.8). Some characterizations of n-FP-projective right R-modules are given in Proposition 3.11. Section 4 is devoted to rings whose every n-FP-projective module is projective. It is shown that wDR ≤ n if and only if every n-FP-projective right R-module is projective if and only if every 0-FP-projective right R-module is of projective ≤ dimension n if and only if every n-FP-projective right R-module has an n- envelope with the unique mapping property (see Theorem 4.1). In particular, R is a right semi-hereditary ring if and only if every 0-FP-projective right R-module has = a monic 0-cover (see Corollary 4.2), and r-dimR wDR if wDR < (see Corollary 4.3). In Section 5, we characterize rings with the finite right -dimension. It is shown that r · -dimR ≤ n if and only if every n-FP-projective right R-module is n + 1- FP-projective if and only if every right R-module with finite FP-injective dimension has FP-injective dimension ≤n (see Theorem 5.1). Section 6 studies how are the rings satisfying that every module is n-FP-projective. It is proven that every right R-module is n-FP-projective if and only ≤ if every right R-module with FP-injective dimension n has an n-cover with the unique mapping property (see Theorem 6.1). We conclude this paper by proving ≤ that rfpDR 1 and 0 is closed under direct products if and only if every right R-module has an epic 0-envelope (see Theorem 6.3). 3. DEFINITION AND GENERAL RESULTS We start with the following definition. Definition 3.1. Let R be a ring and n a nonnegative integer or . A right R- module M is called n-FP-projective provided that Ext1M N = 0 for any right R-module N with FP-idN ≤ n. For a right R-module M, let RM = supnMis n-FP-projective}. We define =− 1 = RM 1ifExtM N 0 for some FP-injective right R-module N. The right -dimension of a ring R, denoted by r-dimR, is defined to be the ≥ = least nonnegative integer n such that RM n implies RM for any right R-module M. If no such n exists, set r-dimR =. Remark 3.2. (1) 0-FP-projective modules were called FP-projective modules in Mao and Ding (2005) and finitely covered modules in Trlifaj (2000). Clearly, finitely presented R-modules are always 0-FP-projective, and projective modules are exactly -FP-projective modules. ≥ (2) It is clear that RM n if and only if M is n-FP-projective for an integer ≥ = ≥ n 0, and RM if and only if M is m-FP-projective for any integer m 0if and only if Ext1M N = 0 for all right R-modules N with FP-idN < . (3) If rFP-dimR ≤ n, then the class of all n-FP-projective right R-modules and the class of all projective right R-modules are the same. Therefore it is always true that r-dimR ≤ rFP-dimR.
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