A theorem on approximate directional derivatives
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Serie 3, Volume 22 (1968) no. 2, p. 343-350 
@article{ASNSP_1968_3_22_2_343_0,
     author = {Bruckner, A. M. and Rosenfeld, Melvin},
     title = {A theorem on approximate directional derivatives},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     publisher = {Scuola normale superiore},
     volume = {Ser. 3, 22},
     number = {2},
     year = {1968},
     pages = {343-350},
     zbl = {0155.10503},
     mrnumber = {247012},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1968_3_22_2_343_0}
}

Bruckner, A. M.; Rosenfeld, M. A theorem on approximate directional derivatives. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Serie 3, Volume 22 (1968) no. 2, pp. 343-350. http://www.numdam.org/item/ASNSP_1968_3_22_2_343_0/

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