@article{MOMA_1972__76_177596,
     author = {Su-Shing Chen},
     title = {The {Schwarz} {Lemma} and the {Hadamard} {Three-Spheres} {Theorem} in {Hilbert} {Spaces.}},
     journal = {Monatshefte f\"ur Mathematik},
     pages = {218--221},
     publisher = {mathdoc},
     volume = {76},
     year = {1972},
     zbl = {0241.58003},
     url = {http://geodesic.mathdoc.fr/item/MOMA_1972__76_177596/}
}