@article{DE_1990_26_3_a16,
     author = {K. B. Sabitov},
     title = {The maximum principle for systems of second-order equations of mixed type},
     journal = {Differencialʹnye uravneni\^a},
     pages = {488--494},
     publisher = {mathdoc},
     volume = {26},
     number = {3},
     year = {1990},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1990_26_3_a16/}
}