1. Hale J.K. and Lunel S.M.V. Introduction to Functional Differential Equations, Springer-Verlag, 1993.
2. Hille E. and Phillips R.S. Functional Analysis and Semi-Groups, American Math. Soc., 1957.
3. Grabosch A. and Moustakas U., A semigroup approach to retarded differential equations One-Parameter Semigroups of Positive Operators, Lecture Notes in Math., 1184 Springer-Verlag, 1986, 219–232.
4. Grabosch A. and Moustakas U., A semigroup approach to retarded equations, ibid., 356–367.
5. Kaashoek M.A. and Lunel S.M.V., Characteristic matrices and spectral properties off evolutionary systems, Trans. Amer. Math. Soc., 334(1992), 479–517.