@article{10_4064_cm_6_1_223_226,
     author = {A. Pil\'s},
     title = {The characteristic equation for partial differential equations of the first order},
     journal = {Colloquium Mathematicum},
     pages = {223--226},
     publisher = {mathdoc},
     volume = {6},
     number = {1},
     year = {1958},
     doi = {10.4064/cm-6-1-223-226},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/cm-6-1-223-226/}
}