@article{SM_1986_53_1_a2,
     author = {B. Yu. Sternin},
     title = {Differential equations of subprincipal type},
     journal = {Sbornik. Mathematics},
     pages = {37--67},
     publisher = {mathdoc},
     volume = {53},
     number = {1},
     year = {1986},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1986_53_1_a2/}
}