Ammar, A., A. Jeribi, and K. Mahfoudhi. “Measure of Noncompactness, Essential Approximation and Defect Pseudospectrum”. Methods of Functional Analysis and Topology 25, no. 1 (March 25, 2019): 1–11. Accessed December 15, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/708.