1. V. A. Pliss, Non-Local Problems of the Theory of Oscillations (Nauka, Moscow, 1964; Academic Press, London, 1966).
2. B. P. Demidovich, Lectures on Mathematical Stability Theory (Nauka, Moscow, 1967) [in Russian].
3. E. A. Coddington and N. Levinson, Theory of Ordinary Differential Equations (McGraw-Hill, New York, 1955; Inostrannaya Literatura, Moscow, 1958).
4. G. A. Leonov, Usp. Mekh. 1(3), 3 (2002).
5. V. V. Nemytskii and V. V. Stepanov, Qualitative Theory of Differential Equations (Gostekhizdat, Moscow, 1947; Princeton Univ. Press, Princeton, N.J., 1960).