@article{ASNSP_1968_3_22_2_343_0,
     author = {Bruckner, A. M. and Rosenfeld, M.},
     title = {A theorem on approximate directional derivatives},
     journal = {Annali della Scuola Normale Superiore di Pisa - Scienze Fisiche e Matematiche},
     pages = {343--350},
     publisher = {Scuola normale superiore},
     volume = {Ser. 3, 22},
     number = {2},
     year = {1968},
     mrnumber = {247012},
     zbl = {0155.10503},
     language = {en},
     url = {http://archive.numdam.org/item/ASNSP_1968_3_22_2_343_0/}
}