1. B. M. Levitan and V. V. Zhikov, Almost Periodic Functions and Differential Equations, Izdat. Moskov. Univ., Moscow, 1978; English translation: Cambridge University Press, Cambridge, 1982.
2. S. G. Krein, Linear Differential Equations in a Banach space, Nauka, Moscow, 1967; English translation: Am. Math. Soc., Providence, R.I., 1971.
3. E. Hille and R. S. Phillips, Functional Analysis and Semi-Groups, Am. Math. Soc., New York, 1957.
4. A. G. Baskakov, “Investigation of spectral properties of the differential operator ℒ =™d/dt+A(t) by means of the semigroup exp ℒt, t≥0,” in: Intern. Conf. on Functional Equations and Applications, Moscow, Russia, August 14–21, 1994, pp. 8–9.
5. J. L. Massera and J. J. Schäffer, Linear Differential Equations and Function Spaces, Academic Press, New York, 1966.