@article{MAN_1960__140_160767,
     author = {L. H\"ORMANDER},
     title = {Differential {Operators} of {Principal} {Type.}},
     journal = {Mathematische Annalen},
     pages = {124--146},
     publisher = {mathdoc},
     volume = {140},
     year = {1960},
     zbl = {0090.08101},
     url = {http://geodesic.mathdoc.fr/item/MAN_1960__140_160767/}
}