KUDASHOV, Z.; KAIYRBEK, O. A. 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, [S. l.], v. 73, n. 2, p. 31–38, 2012. Disponível em: https://bm.kaznu.kz/index.php/kaznu/article/view/134. Acesso em: 3 jul. 2024.