@article{PORMA_1974__33_4_115239,
     author = {Breves Filho, J.A.},
     title = {On the normal systems of differential equations},
     journal = {Portugaliae mathematica},
     pages = {199--201},
     publisher = {mathdoc},
     volume = {33},
     number = {4},
     year = {1974},
     mrnumber = {0369782},
     zbl = {0305.34012},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PORMA_1974__33_4_115239/}
}