@article{TMF_1979_41_2_a10,
     author = {M. F. Sarry},
     title = {Perturbation theory for a~degenerate level},
     journal = {Teoreti\v{c}eska\^a i matemati\v{c}eska\^a fizika},
     pages = {285--288},
     year = {1979},
     volume = {41},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/TMF_1979_41_2_a10/}
}