1. Qualitative Theory of Second-Order Dynamic Systems;Andronov,1973
2. On the center conditions of certain cubic systems;Alwash;Proc. Amer. Math. Soc.,1998
3. Conditions for a centre in a simple class of cubic systems;Collins;Differential Integral Equations,1997
4. Uniformly isochronous centers of polynomial systems in R2;Conti,1994
5. Limit-cycles and rotated vector fields;Duff;Ann. of Math. (2),1953