@article{DAN_1972_206_4_a13,
     author = {V. N. Tulovskii},
     title = {Asymptotic distribution of eigenvalues of differential operators with variable coefficients},
     journal = {Doklady Akademii Nauk},
     pages = {827--830},
     year = {1972},
     volume = {206},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1972_206_4_a13/}
}