[1] Arnol’d,V.I.,Small Denominators and Problems of Stability of Motion in Classical and Celestial Mechanics,Uspekni Mat.Nauk.USSR.18(1963),91-192.
[2] Moser,J.,On the theory of quasi-periodic motions,SIAM.Rev.8(1966),145-172.
[3] Moser,J.,Convergent series expansion of quasi-periodic motions,Math.Ann.,169(1967),136-176.
[4] Giacaglia,G.E.O.,Perturbation Method in Nonlinear Systems,Springer,New York,Berlin,(1972).
[5] Diliberto,S.P.,On Systems of Ordinary Differential Equations,Contributions to the Theory of Nonlinear Oscillations,Vol.I,(1950),1-48.