@article{FORUM_1997__9_2_141791,
     author = {Qihong Fan and M.W. Wong},
     title = {A characterization of some non-elliptic pseudo-differential operators as {Fredholm} operators.},
     journal = {Forum mathematicum},
     pages = {17--28},
     publisher = {mathdoc},
     volume = {9},
     number = {2},
     year = {1997},
     zbl = {0868.35147},
     url = {http://geodesic.mathdoc.fr/item/FORUM_1997__9_2_141791/}
}