@article{DAN_1964_157_4_a7,
     author = {Ling Zhengqiu},
     title = {Perturbation of solutions and perturbation of eigenvalues and eigenfunctions of second-order elliptic equations under},
     journal = {Doklady Akademii Nauk},
     pages = {784--787},
     year = {1964},
     volume = {157},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1964_157_4_a7/}
}