@article{JRAM_1901__123_149113,
     author = {E. Gr\"unfeld},
     title = {Ueber einige in der {Theorie} der linearen {Differentialgleichungen} vorkommende bilineare {Differentialausdr\"ucke.}},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {33--41},
     publisher = {mathdoc},
     volume = {123},
     year = {1901},
     zbl = {32.0328.02},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1901__123_149113/}
}