@article{MAN_1891__38_157540,
     author = {Felix Klein},
     title = {Ueber {Normirung} der linearen {Differentialgleichungen} zweiter {Ordnung.}},
     journal = {Mathematische Annalen},
     pages = {144--152},
     publisher = {mathdoc},
     volume = {38},
     year = {1891},
     zbl = {23.0337.01},
     url = {http://geodesic.mathdoc.fr/item/MAN_1891__38_157540/}
}