O. S. Gretskii, “On the ?-differentiability of mappings of Banach spaces”, Ukr. Mat. Zhurn., vol. 46, no. 10, pp. 1336–1342, Oct. 1994, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5612