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@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/} }
N. V. Medvedev. The nature of conditions for a center. Izvestiâ vysših učebnyh zavedenij. Matematika, no. 6 (1964), pp. 116-122. http://geodesic.mathdoc.fr/item/IVM_1964_6_a14/