@article{MAN_1978__233_163109,
     author = {Ian Knowles},
     title = {On the {Existence} of {Minimal} {Operators} for {Schr\"odinger-Type} {Differential} {Expressions.}},
     journal = {Mathematische Annalen},
     pages = {221--228},
     publisher = {mathdoc},
     volume = {233},
     year = {1978},
     zbl = {0376.35016},
     url = {http://geodesic.mathdoc.fr/item/MAN_1978__233_163109/}
}