TY  - JOUR
AU  - A. P. Sadovskii
TI  - On conditions for a center and focus for nonlinear oscillation equations
JO  - Differencialʹnye uravneniâ
PY  - 1979
SP  - 1716
EP  - 1719
VL  - 15
IS  - 9
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/DE_1979_15_9_a21/
LA  - ru
ID  - DE_1979_15_9_a21
ER  -