@article{ZNSL_1994_218_a1,
     author = {V. M. Babich},
     title = {Mechanical systems, proper oscilations, and spectral theory of selfadjoint operators},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {12--16},
     publisher = {mathdoc},
     volume = {218},
     year = {1994},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1994_218_a1/}
}