@article{DE_1987_23_9_a23,
     author = {I. E. Pleshchinskaya},
     title = {The equivalence of some classes of first-order elliptic and hyperbolic systems and second-order partial differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1634--1637},
     publisher = {mathdoc},
     volume = {23},
     number = {9},
     year = {1987},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1987_23_9_a23/}
}