@article{IVM_1961_4_a5,
     author = {N. V. Medvedev},
     title = {On the problem of center and focus},
     journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
     pages = {57--65},
     publisher = {mathdoc},
     number = {4},
     year = {1961},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1961_4_a5/}
}