1. Introduction to nonlinear mechanics;Krylov,1947
2. Asymptotic methods in the theory of non-linear oscillations;Bogoliubov,1961
3. The geometry of the orbits of artificial satellites;Struble;Arch Rational Mech Anal,1961
4. Perturbation methods;Nayfeh,1973
5. A generalization of the Bogoliubov asymptotic method in the theory of nonlinear oscillation;Popov;Dokl Akad Nauk SSSR,1956