@misc{RM_1989_44_4_a5,
     title = {Joint sessions of the {Petrovskii} {Seminar} on differential equations and mathematical problems of physics and of the {Moscow} {Mathematical} {Society} (twelfth meeting, 18--21 {January~1989)}},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {173--202},
     publisher = {mathdoc},
     volume = {44},
     number = {4},
     year = {1989},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/RM_1989_44_4_a5/}
}