1. A. M. Samoilenko, Elements of the Mathematical Theory of Multifrequency Oscillations. Invariant Tori [in Russian], Nauka, Moscow (1987).
2. Yu. Mozer, “Rapidly convergent iterative method and nonlinear differential controls,” Usp. Mat. Nauk, 23, No. 4, 179–238 (1968).
3. V. A. Eremenko, “On the reduction of a linear system of differential equations having a degenerate matrix with derivatives,” Ukr. Mat. Zh., 32, No. 2, 168–174 (1980).
4. V. A. Eremenko, Investigation of Oscillating Systems with Quasiperiodic Coefficients [in Russian], Author’s Abstract of the Candidate-Degree Thesis (Physics and Mathematics), Kiev (1981).
5. V. L. Kulik and V. A. Eremenko, “On quasiperiodic solutions of a linear system of differential equations having a degenerate matrix with derivatives,” Ukr. Mat. Zh., 32, No. 6, 746–753 (1980).