@article{AFM_1977_3_2_a9,
     author = {Peter Lappan},
     title = {The spherical derivative and normal functions},
     journal = {Annales Fennici Mathematici},
     pages = {301--310},
     publisher = {mathdoc},
     volume = {3},
     number = {2},
     year = {1977},
     doi = {10.5186/aasfm.1977.0312},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.5186/aasfm.1977.0312/}
}