@article{MZ_1982__181_173222,
     author = {Rolf Walter and Norbert Kleinjohann},
     title = {Nonnegativity for the {Curvature} {Operator} and {Isotropy} for {Isometric} {Immersions.}},
     journal = {Mathematische Zeitschrift},
     pages = {129--142},
     year = {1982},
     volume = {181},
     zbl = {0476.53032},
     url = {http://geodesic.mathdoc.fr/item/MZ_1982__181_173222/}
}