@article{1077489095,
author = {Downs, Jr., Thomas L.},
title = {Asymptotic and principal directions at a planar point of a surface},
journal = {Duke Math. J.},
volume = {1},
number = {1},
year = {1935},
pages = { 316-327},
language = {en},
url = {http://dml.mathdoc.fr/item/1077489095}
}
Downs, Jr., Thomas L. Asymptotic and principal directions at a planar point of a surface. Duke Math. J., Tome 1 (1935) no. 1, pp. 316-327. http://gdmltest.u-ga.fr/item/1077489095/