@article{DAN_1957_113_4_a6,
     author = {E. M. Landis and I. G. Petrovskii},
     title = {On the number of limit cycles of equation $\frac{dy}{dx}=\frac{P(x,y)}{Q(x,y)}$, in which~$P$ and~$Q$ 
},
     journal = {Doklady Akademii Nauk},
     pages = {748--751},
     publisher = {mathdoc},
     volume = {113},
     number = {4},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_113_4_a6/}
}