@article{DAN_1976_230_5_a6,
     author = {A. A. Novruzov},
     title = {On the regularity of boundary points for second order equations with nonnegative characteristic form},
     journal = {Doklady Akademii Nauk},
     pages = {1039--1042},
     year = {1976},
     volume = {230},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1976_230_5_a6/}
}