@article{IVM_1980_2_a1,
     author = {L. I. Burkina and G. A. Leonov},
     title = {Frequency conditions for the existence of circular motions and second-order limit cycles in phase systems},
     journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
     pages = {15--21},
     publisher = {mathdoc},
     number = {2},
     year = {1980},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1980_2_a1/}
}