TY  - JOUR
AU  - A. P. Sadovskii
TI  - Conditions for the problem of the center and focus to arise for an $A_3$-system
JO  - Differencialʹnye uravneniâ
PY  - 1990
SP  - 1743
EP  - 1753
VL  - 26
IS  - 10
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/DE_1990_26_10_a11/
LA  - ru
ID  - DE_1990_26_10_a11
ER  -