@article{MAN_1977__231_163057,
     author = {A. Menikoff},
     title = {Pseudo-differential {Operators} with {Double} {Characteristics.}},
     journal = {Mathematische Annalen},
     pages = {145--180},
     publisher = {mathdoc},
     volume = {231},
     year = {1977},
     zbl = {0364.35050},
     url = {http://geodesic.mathdoc.fr/item/MAN_1977__231_163057/}
}