1. Introduction to nonlinear mechanics;Krylov,1947
2. Asymptotic methods in the theory of non-linear oscillations;Bogoliubov,1961
3. A generalization of the Bogoliubov asymptotic method in the theory of nonlinear oscillation;Popov;Dokl Akad Nauk SSSR,1956
4. On the periodic solutions of differential equations obtained by the methods of Poincare and Krylov-Bogoliubov;Bojadziev;Utilitas Mathematica,1973
5. Poincare H. Les methods nouvelles de la mecanique celeste. Paris; 1892.