@article{IVM_1963_6_a12,
     author = {I. S. Kukles and T. Nurov},
     title = {On distinguishing center and focus},
     journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
     pages = {98--108},
     publisher = {mathdoc},
     number = {6},
     year = {1963},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1963_6_a12/}
}