@article{10_4064__27_1_147_161,
     author = {Rainer Felix},
     title = {Differential operators of the first order with degenerate principal symbols},
     journal = {Banach Center Publications},
     pages = {147--161},
     publisher = {mathdoc},
     volume = {27},
     number = {1},
     year = {1992},
     doi = {10.4064/-27-1-147-161},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/-27-1-147-161/}
}