@incollection{AST_1974__19__49_0,
     author = {Bolley, Pierre and Camus, Jacques},
     title = {Hypo-ellipticit\'e partielle et hypo-analyticit\'e d'une classe d'op\'erateurs elliptiques et d\'eg\'en\'er\'es},
     booktitle = {Sur quelques \'equations aux d\'eriv\'ees partielles singuli\`eres},
     series = {Ast\'erisque},
     pages = {49--77},
     publisher = {Soci\'et\'e math\'ematique de France},
     number = {19},
     year = {1974},
     mrnumber = {372421},
     zbl = {0294.35036},
     language = {fr},
     url = {http://archive.numdam.org/item/AST_1974__19__49_0/}
}