@incollection{AST_1973__2-3__197_0,
     author = {Helffer, Bernard and Kannai, Yakar},
     title = {Determining factors and hypoellipticity of ordinary differential operators with double "characteristics"},
     booktitle = {Colloque International C.N.R.S. sur les \'equations aux d\'eriv\'ees partielles lin\'eaires},
     series = {Ast\'erisque},
     pages = {197--216},
     publisher = {Soci\'et\'e math\'ematique de France},
     number = {2-3},
     year = {1973},
     mrnumber = {402260},
     zbl = {0272.34006},
     language = {en},
     url = {http://www.numdam.org/item/AST_1973__2-3__197_0/}
}