TY  - JOUR
AU  - E. K. Ershov
TI  - On the number of cycles of some differential equations on a two-dimensional torus
JO  - Differencialʹnye uravneniâ
PY  - 1991
SP  - 2167
EP  - 2169
VL  - 27
IS  - 12
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/DE_1991_27_12_a20/
LA  - ru
ID  - DE_1991_27_12_a20
ER  -