@article{DAN_1976_230_5_a0,
     author = {V. M. Babich and B. M. Levitan},
     title = {The focussing problem and the asymptotic behavior of the spectral function of the {Laplace{\textendash}Beltrami} operator},
     journal = {Doklady Akademii Nauk},
     pages = {1017--1020},
     year = {1976},
     volume = {230},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1976_230_5_a0/}
}