@article{10_4064_ap_6_3_269_280,
     author = {Z. Kowalski},
     title = {Generalized characteristic directions for a system of differential equations},
     journal = {Annales Polonici Mathematici},
     pages = {269--280},
     publisher = {mathdoc},
     volume = {6},
     number = {3},
     year = {1959},
     doi = {10.4064/ap-6-3-269-280},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap-6-3-269-280/}
}