@article{MZ_1963__82_170184,
     author = {Johann Sonner},
     title = {Universal and special problems.},
     journal = {Mathematische Zeitschrift},
     pages = {200--211},
     publisher = {mathdoc},
     volume = {82},
     year = {1963},
     zbl = {0129.01202},
     url = {http://geodesic.mathdoc.fr/item/MZ_1963__82_170184/}
}