Communicated by J. TrlifajLet C be a semidualizing R-module, where R is a commutative ring. We first introduce the definition of C-cotorsion modules, and obtain the properties of C-cotorsion modules. As applications, we give some new characterizations for perfect rings. Second, we study the Foxby equivalences between the subclasses of the Auslander class and that of the Bass class with respect to C. Finally, we discuss C-cotorsion dimensions and investigate the transfer properties of strongly C-cotorsion dimensions under almost excellent extensions.J. Algebra Appl. Downloaded from www.worldscientific.com by STOCKHOLM UNIVERSITY on 08/27/15. For personal use only.
X. Chen & J. ChenR (F, M ) = 0 for any flat R-module F . The class of cotorsion modules contains all pure-injective (hence injective) modules. An important feature of flat covers (respectively, cotorsion envelopes) is that their kernels (respectively, cokernels) are cotorsion (respectively, flat) by Wakamatsu's Lemmas [21, Sec. 2.1]). The existence of a flat cover and a cotorsion envelope for any module over any associative ring has been proved in [3]. On the other hand, cotorsion (and flat) modules have turned out to be very useful in characterizing rings. From [17], we know some properties about flat cotorsion modules with respect to semidualizing modules and applications in the properties of categories. Inspired by the above works, it is natural to ask what are the properties of cotorsion dimensions with respect to semidualizing modules, and what do they say about the characterizations of rings? The above problem is the main goal of this paper.In Sec. 2, we recall some basic concepts and properties about semidualizing modules and related modules. We introduce the definition of C-cotorsion modules over commutative rings, discuss the properties of C-cotorsion modules and establish the relation between C-cotorsion modules and cotorsion modules. It is proved that an R-module M is C-cotorsion if and only if M ∈ C ⊥1 and Hom R (C, M ) is a cotorsion R-module. Similarly, the relation between strongly C-cotorsion modules and cotorsion modules is given. As applications, we give some new equivalent characterizations of perfect rings. We prove that R is perfect if and only if C ⊥1 ⊆ Cot C (R). And we prove that the class of cotorsion R-modules in the Auslander class A C (R) and the class of C-cotorsion R-modules in the Bass class B C (R) are equivalent under Foxby equivalence. Finally, for a coherent ring R, it is shown that R is perfect if and only if every C-flat C-cotorsion R-module is C-projective if and only if every C-flat R-module is a pure submodule of some C-projective R-module.Section 3 is devoted to investigate the cotorsion dimensions with respect to semidualizing modules, i.e. strongly C-cotorsion dimensions. And we study transfer properties of the class of strongly C-cotorsion dimensions under almost excellent extensions.
Definitions and General ResultsAt the beginning of this section, we recall some known notions and facts needed in the sequel.Let...