[1] A. H. Clifford, G. B. Preston: The Algebraic Theory of Semigroups, vol. 1, 2. Math. Survey 7, Amer. Math. Soc, 1961, 1967.
[2] M. R. Croisot:
Demi-groups inversif et demi-groups reunions de demi-groups simples. Ann. Sci. Ecole Norm. Sup. (3) 70 (1953), 361-379.
MR 0060504
[3] Kenneth M. Kapp:
Green's relations and quasi-ideals. Czech. Math. Journal, 19 (94) 1969, 80-85.
MR 0238972
[5] E. S. Ljapin: Semigroups. Amer. Math. Soc. Translation, Vol. 3, 1963.