@article{DAN_1990_314_5_a11,
     author = {A. H. Hovhannisyan},
     title = {Structure of hyperbolic equations with variable coefficients that},
     journal = {Doklady Akademii Nauk},
     pages = {1072--1075},
     publisher = {mathdoc},
     volume = {314},
     number = {5},
     year = {1990},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1990_314_5_a11/}
}