Garbouj, Zied, and Haïkel Skhiri. “On a New Class of Operators Related to Quasi-Fredholm Operators”. Methods of Functional Analysis and Topology, vol. 26, no. 2, June 2020, pp. 141-66, https://doi.org/https://doi.org/10.31392/MFAT-npu26_2.2020.06.