@article{SM_1983_46_4_a5,
     author = {Yu. V. Egorov},
     title = {Propagation of singularities for solutions of an equation of principal type},
     journal = {Sbornik. Mathematics},
     pages = {527--538},
     publisher = {mathdoc},
     volume = {46},
     number = {4},
     year = {1983},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1983_46_4_a5/}
}