On differentiability of mappings of finite-dimensional domains into Banach spaces
The well-known Stepanov criterion of the differentiability (approximate differentiability) of real functions is generalized to mappings of subsets of $R^n$ n into Banach spaces satisfying the Rieffel sharpness condition, in particular, reflexive Banach spaces. For Banach spaces that do not satisfy the Rieffel sharpness condition, this criterion is not true.
English version (Springer): Ukrainian Mathematical Journal 51 (1999), no. 1, pp 1-10.
Citation Example: Bondar A. V. On differentiability of mappings of finite-dimensional domains into Banach spaces // Ukr. Mat. Zh. - 1999. - 51, № 1. - pp. 3–11.