@article{MAN_1991__291_1_164868,
     author = {Sebastian Walcher},
     title = {On differential equations in normal form.},
     journal = {Mathematische Annalen},
     pages = {293--314},
     publisher = {mathdoc},
     volume = {291},
     number = {1},
     year = {1991},
     zbl = {0754.34032},
     url = {http://geodesic.mathdoc.fr/item/MAN_1991__291_1_164868/}
}