@article{SM_1972_18_2_a2,
     author = {V. N. Tulovskii},
     title = {Asymptotic distribution of eigenvalues of differential equations},
     journal = {Sbornik. Mathematics},
     pages = {191--207},
     year = {1972},
     volume = {18},
     number = {2},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1972_18_2_a2/}
}