1. E. P. Popov and I. P. Pal’tov, Approximate Methods for Studying Nonlinear Automatic Systems (Fizmatgiz, Moscow, 1960) [in Russian].
2. E. P. Popov, The Theory of Nonlinear Automatic Control and Adjustment Systems (Nauka, Moscow, 1979) [in Russian].
3. A. A. Pervozvanskii, A Course in the Theory of Automatic Control (Nauka, Moscow, 1986) [in Russian].
4. H. K. Khalil, Nonlinear Systems (Prentice Hall, Englewood Cliffs, N.J., 2002).
5. I. G. Malkin, Stability of Motion (Nauka, Moscow, 1966) [in Russian].