@article{700856,
     title = {Zum Eigenwertproblem bei analytischen operatorwertigen Funktionen, deren Koeffizienten positive Operatoren sind},
     booktitle = {General Topology and its Relations to Modern Analysis and Algebra},
     series = {GDML\_Books},
     publisher = {Academia Publishing House of the Czechoslovak Academy of Sciences},
     address = {Praha},
     year = {1967},
     pages = {243-245},
     zbl = {0157.21304},
     url = {http://dml.mathdoc.fr/item/700856}
}

Maibaum, G. Zum Eigenwertproblem bei analytischen operatorwertigen Funktionen, deren Koeffizienten positive Operatoren sind, dans General Topology and its Relations to Modern Analysis and Algebra, GDML_Books,  (1967), pp. 243-245. http://gdmltest.u-ga.fr/item/700856/