1. Nemytskii, V.V. and Stepanov, V.V., Kachestvennaya teoriya
differentsial’nykh uravnenii (Qualitative Theory of Differential Equations), Moscow:
Gostekhizdat, 1947.
2. Amel’kin, V.V., Lukashevich, N.A., and Sadovskii, A.P., Nelineinye
kolebaniya v sistemakh vtorogo poryadka (Nonlinear Oscillations in Second-Order
Systems), Minsk: Belarus. Gos. Univ., 1982.
3. Medvedeva, N.B., On the analytical solvability of the problem of
distinguishing between center and focus, Proc. Steklov Inst.
Math., 2006, vol. 254, pp. 11–100.
4. Romanovski, V.G. and Shafer, D.S., The Center
and Cyclicity Problems: A Computational Algebra Approach, Basel:
Birkhäuser, 2010.
5. Yirong Lin, Jibin Li, and Wentao Huang, Planar
Dynamical Systems. Selected Classical Problems, Berlin: de
Gruyter, 2014.