1. R.A. ADAMS, Sobolev spaces, Academic Press, New York 1975.
2. V. BARBU and S.I. GROSSMAN, Asymptotic behavior of linear integrodifferential systems, Trans. Amer. Math. Soc. 173 (1972), 277–288.
3. J.G. BORISOVIC and A.S. TRBABIN, On the Cauchy problem for linear non-homogeneous differential equations with retarded argument, Solviet Math. Doklady 10 (1969), 401–405.
4. J.A. BURNS and T.L. HERDMAN, Adjoint semigroup theory for a Volterra integrodifferential system, Bulletin of the American Mathematical Society 81 (1975), 1099–1102.
5. , Adjoint semigroup theory for a class of functional differential equations, SIAM J. Math. Anal. 7 (1976), 729–745.