@article{DE_1968_4_11_a0,
     author = {K. S. Sibirskii},
     title = {On some questions of the existence of the center},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1931--1938},
     publisher = {mathdoc},
     volume = {4},
     number = {11},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1968_4_11_a0/}
}