@article{DE_1990_26_10_a11,
     author = {A. P. Sadovskii},
     title = {Conditions for the problem of the center and focus to arise for an $A_3$-system},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1743--1753},
     publisher = {mathdoc},
     volume = {26},
     number = {10},
     year = {1990},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1990_26_10_a11/}
}