@article{MZ_1967__96_170679,
     author = {R.A. Hirschfeld},
     title = {On {Hulls} of {Linear} {Operators.}},
     journal = {Mathematische Zeitschrift},
     pages = {216--222},
     publisher = {mathdoc},
     volume = {96},
     year = {1967},
     zbl = {0148.38404},
     url = {http://geodesic.mathdoc.fr/item/MZ_1967__96_170679/}
}