[1] F. V. ATKINSON: 
On relatively regular operators. Acta Sci. Math. Szeged 15 (1953), 38-56. 
MR 0056835 | 
Zbl 0052.12502[2] R. BOULDIN: 
The product of operators with closed range. Tohoku Math. J. 25 (1973), 359-363. 
MR 0326424 | 
Zbl 0269.47002[4] S. R. CARADUS: 
An equational approach to products of relatively regular operators. (submitted to Aequationes Math.). 
MR 0454696 | 
Zbl 0348.47010[5] S. R. CARADUS: 
Operator theory of the pseudo-inverse. Queen's Papers in Pure and Applied Mathematics, No. 38, Quen's University, Kingston, Ont., 1974. 
Zbl 0286.47001[6] C. W. GROETSCH: 
Representations of the generalized inverse. J. Math. Anal. Appl. 49 (1975), 154-157. 
MR 0361877 | 
Zbl 0295.47012[7] J. J. KOLIHA: 
Convergent and stable operators and their generalization. J. Math. Anal. Appl. 43 (1973), 778-794. 
MR 0324447 | 
Zbl 0264.47016[8] J. J. KOLIHA: Convergence of an operator series. Research Report No. 19, Department of Mathematics, University of Melbourne, 1974.
[9] M. Z. NASHED: 
Generalized inverses, normal solvability and iteration for singular operator equations. in Nonlinear Functional Analysis and Applications, L. B. Rall, Ed., Academic Press, New York, 1971, pp. 311-359. 
MR 0275246 | 
Zbl 0236.41015