@article{IVM_1971_7_a5,
     author = {A. Ya. Kovalenko},
     title = {Perturbation of the periodic solutions of differential equations in the case when the spectrum of the monodromy operator of the generating equation contains~1},
     journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
     pages = {44--49},
     publisher = {mathdoc},
     number = {7},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1971_7_a5/}
}