@article{MZ_1987__194_173818,
     author = {Trien Bang Dang},
     title = {Extremal {Maps} of a {Hilbert} {Space} {Equipped} with a {Natural} {Cone.}},
     journal = {Mathematische Zeitschrift},
     pages = {95--98},
     publisher = {mathdoc},
     volume = {194},
     year = {1987},
     zbl = {0589.47048},
     url = {http://geodesic.mathdoc.fr/item/MZ_1987__194_173818/}
}