@article{DE_2001_37_4_a6,
     author = {L. A. Cherkas and I. S. Shchukina},
     title = {A {Special} {Dulac} {Function} for a {Quadratic} {System} on the {Plane}},
     journal = {Differencialʹnye uravneni\^a},
     pages = {481--487},
     publisher = {mathdoc},
     volume = {37},
     number = {4},
     year = {2001},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_2001_37_4_a6/}
}