@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}, language = {fr}, url = {http://archive.numdam.org/item/JEDP_1976____A9_0/} }