1. Ya. V. Bykov, About Some Problems in the Theory of Integro-Differential Equations (Kirg. Gos. Univ., Frunze, 1957) [in Russian].
2. L. Levitov, Green’s Functions in Problems (Princeton Univ. Press, Princeton, 2011).
3. E. C. Titchmarsh, Eigenfunction Expansions Associated with Second-Order Differential Equations, Part 1 (Princeton Univ. Press, Princeton, 1962).
4. A. I. Botashaev, Periodic Solutions of Volterra Integro-Differential Equations (Mosk. Fiz.-Tekh. Inst., Moscow, 1998) [in Russian].
5. Ya. V. Bykov and D. Ruzikulov, Periodic Solutions of Integro-Differential Equations and Their Asymptotics (Ilim, Frunze, 1986) [in Russian].