1. V. V. Nemytskii and V. V. Stepanov, Qualitative Theory of Differential Equations [in Russian], Gostekhizdat, Moscow-Leningrad (1949).
2. V. I. Arnol'd, "Local problems of analysis," Vestnik MGU, Seriya Matem.,2, 52?56 (1970).
3. I. G. Petrovskii and E. M. Landis, "On the number of limit cycles of the equation dy/dx = P(x,y)/Q(x,y), where P and Q are second degree polynomials," Matem. Sb.,38, 209?250 (1955).
4. Yu. S. Il'yashenko, "The origin of limit cycles under perturbations of the equation dw/dz = ? Rz/Rw, where R(z, w) is a polynomial," Matem. Sb.,78, 360?373 (1969).