TY  - JOUR
AU  - A. P. Sadovskii
TI  - Formal transformations and the problem of distinguishing the center and focus
JO  - Differencialʹnye uravneniâ
PY  - 1978
SP  - 2185
EP  - 2189
VL  - 14
IS  - 12
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/DE_1978_14_12_a10/
LA  - ru
ID  - DE_1978_14_12_a10
ER  -