1. N. L. Lazetic, Differ. Equations 42, 1134–1139 (2006) [Differ. Uravn. 42, 1072–1077 (2006)].
2. A. M. Nakhushev, Equations of Mathematical Biology (Vysshaya Shkola, Moscow, 1995) [in Russian].
3. M. T. Dzhenaliev, On the Theory of Linear Boundary Value Problems for Loaded Differential Equations (Inst. Teor. Prikl. Mat., Almaty, 1995) [in Russian].
4. S. Ya. Yakubov, Linear Differential Operator Equations and Applications (Elm, Baku, 1985) [in Russian].