@incollection{AST_1973__2-3__371_0,
     author = {Derridj, M. and Zuily, C.},
     title = {R\'egularit\'e analytique et {Gevrey,} pour des classes d'op\'erateurs elliptiques paraboliques d\'eg\'en\'er\'es du second ordre},
     booktitle = {Colloque International C.N.R.S. sur les \'equations aux d\'eriv\'ees partielles lin\'eaires},
     series = {Ast\'erisque},
     pages = {371--381},
     publisher = {Soci\'et\'e math\'ematique de France},
     number = {2-3},
     year = {1973},
     mrnumber = {393788},
     zbl = {0303.35027},
     language = {fr},
     url = {http://www.numdam.org/item/AST_1973__2-3__371_0/}
}