@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/}
}