TY  - JOUR
AU  - V. V. Amel'kin
TI  - Letter to the editors: ``On the question of the isochrony of the center of two-dimensional analytic differential systems'' (Differencial'nye Uravnenija \textbf{13} (1977), no.~6, 971--980)
JO  - Differencialʹnye uravneniâ
PY  - 1977
SP  - 1537
VL  - 13
IS  - 8
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/DE_1977_13_8_a28/
LA  - ru
ID  - DE_1977_13_8_a28
ER  -