1. R. Bellman and K. L. Cooke, Differential-Difference Equations, Academic Press, New York (1963).
2. N. N. Krasovskii, Some Problems of the Theory of Stability of Motion [in Russian], Nauka, Moscow (1959).
3. D. I. Martynyuk and A. M. Samoilenko, “On periodic solutions of nonlinear systems with delay,” Mat. Fiz., Issue 3, 128–145 (1967).
4. Yu. A. Mitropol’skii and D. I. Martynyuk, Periodic and Quasiperiodic Oscillations of Systems with Delay [in Russian], Vyshcha Shkola, Kiev (1979).
5. Yu. A. Mitropol’skii and V. I. Fodchuk, “Asymptotic methods of nonlinear mechanics for nonlinear differential equations with retarded argument,” Ukr. Mat. Zh., 18, No. 3, 65–84 (1966).