1. Yu. A. Mitropol'skii,Method of Averaging in the Nonlinear Mechanics [in Russian], Naukova Dumka, Kiev (1971).
2. A.M. Samoilenko, ?Method of averaging in systems with pulses,?Mat. Fizika, Issue 9, 101?117 (1971).
3. A.M. Samoilenko and N. A. Perestyuk,Differential Systems with Pulse Effects [in Russian], Vyshcha Shkola, Kiev (1987).
4. A. F. Filippov,Differential Equations with Discontinuous Right-Hand Sides [in Russian], Nauka, Moscow (1985).
5. Yu. I. Neimark and L. P. Shil'nikov, ?On application of a small parameter to systems of differential equations with discontinuous right-hand sides,?Izv. Akad. Nauk SSSR. Mekh. Mashinostr., No. 6, 51?59 (1959).