@article{GDZPPN00228328X, title = {Separation der Variablen in HILBERTschen R\"aumen.}, journal = {Mathematische Annalen}, volume = {125}, year = {1952}, pages = {401-434}, zbl = {0050.11801}, url = {http://dml.mathdoc.fr/item/GDZPPN00228328X} }
Cordes, H.O. Separation der Variablen in HILBERTschen Räumen.. Mathematische Annalen, Tome 125 (1952) pp. 401-434. http://gdmltest.u-ga.fr/item/GDZPPN00228328X/