[1] Apostol, C. (1985). The reduced minimum modulus. Michigan Mathematical Journal, 32(3), 279-294.
[2] Bouldin, R. (1981). The essential minimum modulus. Indiana University Mathematics Journal, 30(4), 513-517.
[3] Chen, G., Xue, Y. (1998). The expression of the generalized inverse of the perturbed operator under Type I perturbation in Hilbert spaces. Linear algebra and its applications, 285(1-3), 1-6.
[4] Deng, C. Y., Du, H. K. (2006). The reduced minimum modulus of Drazin inverses of linear operators on Hilbert spaces. Proceedings of the American Mathematical Society, 134(11), 3309-3317.
[5] Moorthy, C. G., Ramasamy, C. T. Unbounded Multiplication Operators With Closed Range.
[6] Moorthy, C. G., Johnson, P. S. (2004). Closed range multiplication operators. Mathematical Analysis and Applications, 133-138.
[7] HA, J., RAKOCEVIo, V. (1998). Continuity of the Drazin inverse II. Studia Mathematica, 131, 2.
[8] Kulkarni, S. H., Nair, M. T. (2000). A characterization of closed range operators. Indian Journal of Pure and Applied Mathematics, 31(4), 353-362.
[9] Kulkarni, S. H., Nair, M. T., Ramesh, G. (2008). Some properties of unbounded operators with closed range. Proceedings Mathematical Sciences, 118, 613-625.
[10] Laursen, K. B., Neumann, M. (2000). An introduction to local spectral theory (No. 20). Oxford University Press.
[11] Mbekhta, M. (1993). Generalized spectrum and a problem of Apostol. Proceedings of the American Mathematical Society, 857-859.
[12] Rudin, W. Real and complex analysis McGraw-Hill, New York, 1974. MR, 49, 8783.
[13] Schmoeger, C. (1994). Perturbation properties of some classes of operators. Rend. Math. Appl., 7, 533-541.
[14] Taylor, A. E., Lay, D. C. (1986). Introduction to functional analysis. Krieger Publishing Co., Inc.
