1. A. M. Samoilenko and N. A. Perestyuk,Differential Equations with Pulse Influence [in Russian], Vyshcha Shkola, Kiev (1987).
2. A. A. Boichuk,Constructive Methods in the Analysis of Boundary-Value Problems [in Russian], Naukova Dumka, Kiev (1990).
3. A. M. Samoilenko and A. A. Boichuk, ?Linear Noetherian boundary-value problems for differential systems with pulse influence,?Ukr. Mat. Zh.,44, No 3, 582?586 (1992).
4. A. A. Boichuk, N. A. Perestyuk, and A. M. Samoilenko, ?Periodic solutions of momentum differential systems for critical cases,?Diff. Uravn., No 9, 1516?1521 (1991).
5. I. G. Malkin,Some Problems in the Theory of Nonlinear Oscillations [in Russian], Gostekhisdat, Moscow (1956).