@article{DAN_1993_330_6_a4,
     author = {P. I. Plotnikov},
     title = {Equations with a variable direction of parabolicity and the},
     journal = {Doklady Akademii Nauk},
     pages = {691--693},
     year = {1993},
     volume = {330},
     number = {6},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1993_330_6_a4/}
}