1. The center problem for a family of systems of differential equations having a nilpotent singular point,;A. Algaba;J. Math. Anal. Appl.,2008
2. Local bifurcation of limit cycles and integrability of a class of nilpotent systems of differential equations,;A. Algaba;Appl. Math. Comput.,2009
3. Nonlinear Oscillations in Second-Order Systems,;V. V. Amel'kin;Minsk,1982
4. Solution of the problem of the center and the focus in one case (Russian),;A. Andreev;Akad. Nauk SSSR. Prikl. Mat. Meh.,1953
5. Investigation on the behaviour of the integral curves of a system of two differential equations in the neighborhood of a singular point,;A. Andreev;Translations Amer. Math. Soc.,1958