1.Amini, A., Ershad, M. and Sharif, H., Rings over which flat covers of finitely generated modules are projective, Comm. Algebra 36 (8) (2008), 2862–2871.
2.Stenström, B., Flatness and localization over monoids, Math. Nachr. 48 (1971), 315–334.
3.Enochs, E. E., Jenda, O. M. G. and Lopez-Ramos, J. A., The existence of Gorenstein flat covers, Math. Scand. 94 (2004), 46–62.
4.Enochs, E. E. and Oyonarte, L., Covers, envelopes and cotorsion theories (Nova Science, New York, 2002).
5.Fountain, J., Perfect semigroups, Proc. Edinb. Math. Soc. 20 (2) (1976), 87–93.
6.Howie, J. M., Fundamentals of semigroup theory. London Mathematical Society Monograph (London Mathematical Society, London, 1995)
7.Isbell, J., Perfect monoids, Semigroup Forum 2 (1971), 95–118.
8.Kilp, M., Perfect monoids revisited, Semigroup Forum 53 (1996), 225–229.
9.Kilp, M. and Laan, V., On flatness properties of cyclic acts, Comm. Algebra 28 (6) (2000), 2919–2926.
10.Kilp, M., On monoids over which all strongly flat cyclic right acts are projective, Semigroup Forum 52 (1996), 241–245.
11.Kilp, M., Knauer, U., Mikhalev, A. V., Monoids, acts and categories, in De Gruyter expositions in mathematics, Vol. 29, (de Gruyter, Berlin, 2000).
12.Mao, L. and Ding, N., Envelopes and covers by modules of finite FP-injective and flat dimensions, Comm. Algebra 35 (3) (2007), 833–849.
13.Mahmoudi, M. and Renshaw, J., On covers of cyclic acts over monoids. Semigroup Forum 77 (2008), 325–338.
14.Laan, V., On a generalization of strong flatness. Acta Comment, Univ. Tartu. Math. 2 (1998), 55–60.
15.Xu, J., Flat covers of modules, in Lecture Notes in Mathematics, Vol. 1634 (Springer, Berlin, 1996).