1. A. Algaba, E. Frejre, E. Gamero, Isochronicity via normal form
2. Asymptotic Behavior and Stability Problems in Ordinary Differential Equations;Cesari,1963
3. Isochronous centers in planar polynomial systems;Christopher;SIAM J. Math. Anal.,1997
4. C. J. Christopher, J. Devlin, N. G. Lloyd, On the classification of Liénard systems with amplitude- independent periods
5. J. Chavarriga, J. Giné, I. Garcı́a, Isochronous centers of cubic systems with degenerate infinity, Differential Equations Dynam. Systems