González, M., A. Martínez-Abejón, and M. Salas-Brown. “Perturbation Classes for Semi-Fredholm Operators on Subprojective and Superprojective Spaces”. Annales Fennici Mathematici, vol. 36, no. 2, Aug. 2011, pp. 481-9, doi:10.5186/aasfm.2011.3625.