Gretskii, O. S. “On the ?-Differentiability of Mappings of Banach Spaces”.
Ukrains’kyi Matematychnyi Zhurnal
46, no. 10 (October 25, 1994): 1336–1342. Accessed April 11, 2025.
https://umj.imath.kiev.ua/index.php/umj/article/view/5612
.