Let be an abstract class (closed under isomorpic copies) of left -modules. In the first part of the paper some sufficient conditions under which is a precover class are given. The next section studies the -precovers which are -covers. In the final part the results obtained are applied to the hereditary torsion theories on the category on left -modules. Especially, several sufficient conditions for the existence of -torsionfree and -torsionfree -injective covers are presented.
Recently, Rim and Teply , using the notion of -exact modules, found a necessary condition for the existence of -torsionfree covers with respect to a given hereditary torsion theory for the category -mod of all unitary left -modules over an associative ring with identity. Some relations between -torsionfree and -exact covers have been investigated in . The purpose of this note is to show that if is Goldie’s torsion theory and is a precover class, then is a precover class whenever...
It is a well-known fact that modules over a commutative ring in general cannot be classified, and it is also well-known that we have to impose severe restrictions on either the ring or on the class of modules to solve this problem. One of the restrictions on the modules comes from freeness assumptions which have been intensively studied in recent decades. Two interesting, distinct but typical examples are the papers by Blass [1] and Eklof [8], both jointly with Shelah. In the first case the authors...
Let k be a commutative field. For any a,b∈ k, we denote by the deformation of the 2-dimensional Weyl algebra over k associated with the Jordanian Hecke symmetry with parameters a and b. We prove that: (i) any can be embedded in the usual Weyl algebra A₂(k), and (ii) is isomorphic to A₂(k) if and only if a = b.
Let n ≥ 3 be a positive integer. We study symmetric skew n-derivations of prime and semiprime rings and prove that under some certain conditions a prime ring with a nonzero symmetric skew n-derivation has to be commutative.