TY  - JOUR
AU  - A. P. Sadovskii
TI  - The problem of distinguishing the center and the focus
JO  - Differencialʹnye uravneniâ
PY  - 1973
SP  - 650
EP  - 659
VL  - 9
IS  - 4
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/DE_1973_9_4_a8/
LA  - ru
ID  - DE_1973_9_4_a8
ER  -