[1] BASS H.: Finitistic dimension and a homological characterization of semi-primary rings. Trans. Amer. Math. Soc. 95 (1960), 466-488. MR 0157984

[2] BICAN L.: A note on pure subgroups. Contributions to General Algebra 12, Proceedings of the Vienna Conference, June 3-6, 1999, Verlag Johannes Heyn, Klagenfurt 2000, pp. 105-107. MR 1777650

[3] BICAN L.: Pure subgroups. Math. Bohem. 126 (2001), 649-652. MR 1970267 | Zbl 0983.20054

[4] BICAN L.-EL BASHIR R.-ENOCHS E.: All modules have flat covers. Proc. London Math. Soc. 33 (2001), 649-652. MR 1832549

[5] BICAN L.-TORRECILLAS B.: Precovers. Czechoslovak Math. J. 53(128) (2003), 191-203. MR 1962008 | Zbl 1184.16002

[6] BICAN L.-TORRECILLAS B.: On covers. J. Algebra 236 (2001), 645-650. MR 1813494 | Zbl 0973.16002

[7] ECKMANN B.-SCHOPF A.: Über injektive Moduln. Arch. Math. (Basel) 4(2) (1953), 75-78. MR 0055978 | Zbl 0050.25904

[8] ENOCHS E.: Infective and flat covers, envelopes and resolvents. Israel J. Math. 39 (1981), 189-209. MR 0636889

[9] FUCHS L.: Infinite Abelian Groups, Vol I; II. Academic Press, New York, 1973; 1977. MR 0255673

[10] TEPLY M. L.: Torsion-free covers II. Israel J. Math. 23 (1976), 132-136. MR 0417245 | Zbl 0321.16014

[11] XU J.: Flat Covers of Modules. Lecture Notes in Math. 1634, Springer Verlag, Berlin- Heidelberg-New York, 1996. MR 1438789 | Zbl 0860.16002