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