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@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/} }
H.G. HELFENSTEIN. On the Distortion of Conformal Immersions between 2-Space Forms.. Mathematische Annalen, Tome 183 (1969), pp. 159-168. http://geodesic.mathdoc.fr/item/MAN_1969__183_161887/