@incollection{JEDP_1993____A1_0,
     author = {B\'ethuel, Fabrice and Ghidaglia, Jean-Michel},
     title = {R\'egularit\'e des solutions de certaines \'equations elliptiques en dimension deux et formule de la co-aire},
     booktitle = {},
     series = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {1},
     pages = {1--36},
     publisher = {Ecole polytechnique},
     year = {1993},
     mrnumber = {94k:35044},
     zbl = {0812.35037},
     language = {fr},
     url = {http://archive.numdam.org/item/JEDP_1993____A1_0/}
}