@article{700609,
     title = {On the geometric characterization of differentiability},
     booktitle = {General topology and its relations to modern analysis and algebra IV},
     series = {GDML\_Books},
     publisher = {Society of Czechoslovak Mathematicians and Physicist},
     address = {Praha},
     year = {1977},
     pages = {113-115},
     mrnumber = {MR0426019},
     zbl = {0372.46041},
     url = {http://dml.mathdoc.fr/item/700609}
}

Durdil, J. On the geometric characterization of differentiability, dans General topology and its relations to modern analysis and algebra IV, GDML_Books,  (1977), pp. 113-115. http://gdmltest.u-ga.fr/item/700609/