1. Poincaré, H., Sur les courbes
définies par des équations différentielles. Translated
under the title: O krivykh, opredelyayemykh differentsial’nymi
uravneniyami, Moscow–Leningrad: GITTL, 1947.
2. Lyapunov, A.M., Obshchaya zadacha ob ustoichivosti
dvizheniya (General Problem of Motion Stability), Moscow–Leningrad:
GITTL, 1950.
3. Sadovskii, A.P., On the problem of distinguishing between center and focus
for systems with nonzero linear part, Differ. Uravn.,
1976, vol. 12, no. 7, pp. 1238–1246.
4. Il’yashenko, Yu.S., Algebraic nonsolvability and almost algebraic solvability of
the center–focus problem, Funct. Anal. Appl., 1972,
vol. 6, no. 3, pp. 197–202.
5. Űola˛dek, H. and Llibre, J., The Poincaré center problem,
J. Dyn. Control Syst., 2008, vol. 14, no. 4, pp. 505–535.