@article{IVM_1964_6_a14,
     author = {N. V. Medvedev},
     title = {The nature of conditions for a center},
     journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
     pages = {116--122},
     publisher = {mathdoc},
     number = {6},
     year = {1964},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1964_6_a14/}
}