Y. V. . Bogdansky, “The maximum principle for irregular elliptic differential equation in the countable-dimensional Hilbert space ”, Ukr. Mat. Zhurn., vol. 40, no. 1, pp. 21–25, Dec. 1987, Accessed: Feb. 17, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/9048