@article{GDZPPN00207768X, title = {Ungel\"oste Probleme.}, journal = {Elemente der Mathematik}, volume = {26}, year = {1971}, pages = {58-58}, url = {http://dml.mathdoc.fr/item/GDZPPN00207768X} }
Hadwiger, H.; Schneider, R. Ungelöste Probleme.. Elemente der Mathematik, Tome 26 (1971) p. 58. http://gdmltest.u-ga.fr/item/GDZPPN00207768X/