1. A. Wintner, Amer. J. Math.,68, 553?559 (1946).
2. J. J. Schäffer, Bull. Amer. Math. Soc.,70, 243?245 (1964).
3. J. L. Massera and J. J. Schäffer, Linear Differential Equations and Function Spaces, Acad. Press, New York (1966).
4. L. V. Kantorovich, UMN,3, 6, 89?185 (1948).
5. M. G. Krein, DAN SSSR,144, No. 3, 268?272 (1962).