@article{MZ_1968__105_170963,
     author = {S. Hildebrandt and B. Schmidt},
     title = {Zur {Konvergenz} von {Operatorprodukten} im {Hilbertraum.}},
     journal = {Mathematische Zeitschrift},
     pages = {62--71},
     publisher = {mathdoc},
     volume = {105},
     year = {1968},
     zbl = {0164.44003},
     url = {http://geodesic.mathdoc.fr/item/MZ_1968__105_170963/}
}