@article{GAMA_1976_1976_1_a3,
     author = {Gonz\'alez, M. O.},
     title = {A spherical representation of the real directional derivative.},
     journal = {Gaceta matem\'atica},
     pages = {25-29},
     publisher = {mathdoc},
     volume = {28},
     number = {1-2},
     year = {1976},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/GAMA_1976_1976_1_a3/}
}