@article{ZNSL_1978_78_a0,
     author = {V. M. Babich and B. M. Levitan},
     title = {Problem of focusing and the asymptotics of the spectral function of the {Laplace--Beltrami} {operator.~I}},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {3--19},
     publisher = {mathdoc},
     volume = {78},
     year = {1978},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1978_78_a0/}
}