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2. A. M. Samoilenko and N. A. Perestyuk, Impulsive Differential Equations [in Russian], Vyshcha Shkola, Kiev (1987).
3. A. F. Filippov, Differential Equations with Discontinuous Right-Hand Sides [in Russian], Nauka, Moscow (1985).
4. A. A. Andronov, A. A. Vitt, and S. É. Khaikin, Theory of Oscillations [in Russian], Fizmatlit, Moscow (1959).
5. N. V. Butenin, Yu. I. Neimark, and N. A. Fufaev, Introduction to the Theory of Nonlinear Oscillations [in Russian], Nauka, Moscow (1987).