V. V. Ostapenko, “To the problem of continuity of many-valued mappings”, Ukr. Mat. Zhurn., vol. 47, no. 11, pp. 1519–1525, Nov. 1995, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5542