@incollection{JEDP_1976____A9_0,
     author = {Helffer, Bernard},
     title = {Addition de variables et application \`a la r\'egularit\'e},
     booktitle = {},
     series = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {9},
     pages = {1--12},
     publisher = {Ecole polytechnique},
     year = {1976},
     doi = {10.5802/jedp.144},
     language = {fr},
     url = {https://www.numdam.org/articles/10.5802/jedp.144/}
}