TY  - JOUR
AU  - È. F. Kirnitskaya
AU  - K. S. Sibirskii
TI  - Conditions for two centers for a quadratic differential system
JO  - Differencialʹnye uravneniâ
PY  - 1978
SP  - 1589
EP  - 1593
VL  - 14
IS  - 9
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/DE_1978_14_9_a5/
LA  - ru
ID  - DE_1978_14_9_a5
ER  -