@incollection{JEDP_1985___1_A7_0,
     author = {Xu, Chaojiang},
     title = {Hypoellipticit\'e d'\'equations aux d\'eriv\'ees partielles non lin\'eaires},
     booktitle = {},
     series = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {7},
     pages = {1--16},
     publisher = {Ecole polytechnique},
     number = {1},
     year = {1985},
     zbl = {0586.35024},
     language = {fr},
     url = {http://archive.numdam.org/item/JEDP_1985___1_A7_0/}
}