@article{MAN_1969__183_161887,
     author = {H.G. HELFENSTEIN},
     title = {On the {Distortion} of {Conformal} {Immersions} between {2-Space} {Forms.}},
     journal = {Mathematische Annalen},
     pages = {159--168},
     publisher = {mathdoc},
     volume = {183},
     year = {1969},
     zbl = {0172.46501},
     url = {http://geodesic.mathdoc.fr/item/MAN_1969__183_161887/}
}