@article{MZ_1973__134_171988,
     author = {E. jr. Sperner},
     title = {Zur {Symmetrisierung} von {Funktionen} auf {Sph\"aren.}},
     journal = {Mathematische Zeitschrift},
     pages = {317--328},
     publisher = {mathdoc},
     volume = {134},
     year = {1973},
     zbl = {0283.26015},
     url = {http://geodesic.mathdoc.fr/item/MZ_1973__134_171988/}
}