@article{MZ_1959__72_183599,
     author = {Einar Hille},
     title = {An {Application} of {Pr\"ufer's} {Method} to a {Singular} {Boundary} {Value} {Problem.}},
     journal = {Mathematische Zeitschrift},
     pages = {95--106},
     publisher = {mathdoc},
     volume = {72},
     year = {1959},
     zbl = {0085.30302},
     url = {http://geodesic.mathdoc.fr/item/MZ_1959__72_183599/}
}