1. A. M. Lyapunov, in Collected Works (Izd. Akad. Nauk SSSR, Moscow, 1956), Vol. 2 [in Russian].
2. Yu. N. Bibikov, Local Problems of the Theory of Multifrequency Nonlinear Oscillations (Izd. St. Petersburg. Univ., St. Petersburg, 2003) [in Russian].