@incollection{JEDP_1981____A10_0, author = {Charles Fefferman and D. H. Phong}, title = {Pseudo-differential operators with symbols admitting negative values}, booktitle = {}, series = {Journ\'ees \'equations aux d\'eriv\'ees partielles}, eid = {10}, pages = {1--6}, publisher = {\'Ecole polytechnique}, year = {1981}, zbl = {0482.35081}, language = {en}, url = {https://proceedings.centre-mersenne.org/item/JEDP_1981____A10_0/} }