@article{MZ_1987__196_173874,
     author = {Giovanni Dore and Alberto Venni},
     title = {On the {Closedness} of the {Sum} of {Two} {Closed} {Operators.}},
     journal = {Mathematische Zeitschrift},
     pages = {189--202},
     year = {1987},
     volume = {196},
     zbl = {0615.47002},
     url = {http://geodesic.mathdoc.fr/item/MZ_1987__196_173874/}
}