@article{GDZPPN002320533, title = {Universal Families of C$^\circ$$^\circ$-Functions on D2.}, journal = {Mathematische Annalen}, volume = {255}, year = {1981}, pages = {67-84}, zbl = {0442.58006}, url = {http://dml.mathdoc.fr/item/GDZPPN002320533} }
Jänich, Klaus. Universal Families of C°°-Functions on D2.. Mathematische Annalen, Tome 255 (1981) pp. 67-84. http://gdmltest.u-ga.fr/item/GDZPPN002320533/