Ali, Dalia S., and Buthainah A.A. Ahmed. “Perturbation of Weyl’s Theorems for Unbounded  Upper Triangular Operator Matrices”. Ibn AL-Haitham Journal For Pure and Applied Sciences, vol. 38, no. 4, Oct. 2025, pp. 287-94, https://doi.org/10.30526/38.4.3473.