@article{CMH_1959__33_139172,
     author = {Olli Lehto},
     title = {The spherical derivative of meromorphic functions in the neighbourhood of an isolated singularity.},
     journal = {Commentarii mathematici Helvetici},
     pages = {196--205},
     publisher = {mathdoc},
     volume = {33},
     year = {1959},
     zbl = {0086.06402},
     url = {http://geodesic.mathdoc.fr/item/CMH_1959__33_139172/}
}