Kudashov, Zh, and O A Kaiyrbek. “On Biorthogonal Property of Systems of Root Functions of the Neumann Problem for the Perturbation of the Integral Boundary Condition”. Journal of Mathematics, Mechanics and Computer Science 73, no. 2 (June 28, 2012): 31–38. Accessed July 3, 2024. https://bm.kaznu.kz/index.php/kaznu/article/view/134.