@article{MAN_1970__188_162050,
     author = {V. ISTRATESCU},
     title = {On {Some} {Classes} of {Operators.}},
     journal = {Mathematische Annalen},
     pages = {227--232},
     publisher = {mathdoc},
     volume = {188},
     year = {1970},
     zbl = {0188.19702},
     url = {http://geodesic.mathdoc.fr/item/MAN_1970__188_162050/}
}