[1] J. Adamek, H. Herrlich, and G.E. Strecker, \Abstract and Concrete Categories", John Wiley and Sons, Inc., 1990.
[2] F.W.. Anderson and K.R. Fuller, \Rings and Categories of Modules", Springer, New York, 1974.
[3] B. Banaschewski, Injectivity and essential extensions in equational classes of algebras, Queen's Paper in Pure Appl. Math. 25 (1970), 131-147.
[4] H. Barzegar and M.M. Ebrahimi, Sequentially pure monomorphisms of acts over semigroups, Eur. J. Pure Appl. Math. 1(4) (2008), 41-55.
[5] H. Barzegar, M.M. Ebrahimi, and M. Mahmoudi, Essentiality and injectivity relative to sequential purity of acts, Semigroup Forum 79(1) (2009), 128-144.
[6] P. Berthiaume, The injective envelope of S-Sets, Canad. Math. Bull. 10(2) (1967), 261-273.
[7] K.R. Bhutani, Injectivity and injective hulls of abelian groups in a localic topos, Bull.
Austral. Math. Soc. 37 (1988), 43-59.
[8] S. Burris and H.P. Sankapanavar, \A Course in Universal Algebra", Graduate Texts in Math. No. 78, Springer-Verlag, 1981.
[9] D. Dikranjan and W. Tholen, \Categorical Structure of Closure Operators, with Applications to Topology, Algebra, and Discrete Mathematics", Mathematics and its Applications, Kluwer Academic Publisher, 1995.
[10] M. M. Ebrahimi, M. Haddadi, and M. Mahmoudi, Injectivity in a category: An overview of well behaviour theorems, Algebra, Groups, and Geometries 26 (2009), 451-472.
[11] M.M. Ebrahimi, Algebra in a Grothendieck Topos: Injectivity in quasi-equational classes, J. Pure Appl. Algebra 26 (1982), 269-280.
[12] M.M. Ebrahimi, Equational compactness of sheaves of algebras on a Noetherian locale, Algebra Universalis 16 (1983), 318-330.
[13] M.M. Ebrahimi, Internal completeness and injectivity of Boolean algebras in the topos of M-sets, Bull. Austral. Math. Soc. 41(2) (1990), 323-332.
[14] M.M. Ebrahimi, On ideal closure operators of M-sets, Southeast Asian Bull. Math. 30 (2006), 439-444.
[15] M.M. Ebrahimi and M. Mahmoudi, Purity and equational compactness of projection algebras, Appl. Categ. Structures 9(4) (2001), 381-394.
[16] M.M. Ebrahimi and M. Mahmoudi, The category of M-sets, Italian J. Pure Appl. Math. 9 (2001), 123-132.
[17] M.M. Ebrahimi and M. Mahmoudi, Baer criterion and injectivity of projection algebras, Semigroup Forum 71(2) (2005), 332-335.
[18] M.M. Ebrahimi, M. Mahmoudi, and Gh. Moghaddasi Angizan, Injective hulls of acts over left zero semigroups, Semigroup Forum 75(1) (2007), 212-220.
[19] M.M. Ebrahimi, M. Mahmoudi, and Gh. Moghaddasi Angizan, On the Baer criterion for acts over semigroups, Comm. Algebra 35(12) (2007), 3912-3918.
[20] M.M. Ebrahimi, M. Mahmoudi, and L. Shahbaz, Proper behaviour of sequential injectivity of acts over semigroups, Comm. Algebra 37(7) (2009), 2511-2521.
[21] R. Goldblatt, \Topoi: The Categorial Analysis of Logic", North Holland, 1986.
[22] M. Kilp, U. Knauer, and A. Mikhalev, \Monoids, Acts and Categories", Walter de Gruyter, Berlin, New York, 2000.
[23] S. Maclane, \Categories for theWorking Mathematicians", Graduate Texts in Mathematics, No. 5, Springer-Verlag, 1971.
[24] M. Mahmoudi, Internal injectivity of Boolean algebras in MSet, Algebra Universalis 41 (1999), 155-175.
[25] M. Mahmoudi and Gh. Moghaddasi Angizan, Injective hulls of acts over idempotent semigroups, Semigroup Forum 74(2) (2007), 240-246.
[26] M. Mahmoudi and L. Shahbaz, Sequentially dense essential monomorphisms of acts over semigroups, Appl. Categ. Structures 18(5) (2010), 461-471.
[27] M. Mahmoudi and L. Shahbaz, Characterizing semigroups by sequentially dense injective acts, Semigroup Forum 75(1)(2007), 116-128.
[28] W. Taylor, Residually small varieties, Algebra Universalis 2 (1972), 33-53.
[29] B.R. Tennison, \Sheaf Theory", Cambridge University Press, 1975.
[30] W. Tholen, Injective Objects and Cogenerating sets, J. Algebra 73(1) (1981), 139-155.
[31] W. Wechiler, \Universal Algebra for Computer Scientists", EATCS Monographs on Theoretical Computer Science, Springer-Verlag, 1992.