Dmytryshyn, M.I., and O.V. Lopushansky. “Spectral Approximations of Strongly Degenerate Elliptic Differential Operators”. Carpathian Mathematical Publications 11, no. 1 (June 30, 2019): 48–53. Accessed November 24, 2024. https://journals.pnu.edu.ua/index.php/cmp/article/view/1507.