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2. Andreev, A.F., Vvedenie v lokal’nuyu kachestennuyu teoriyu differentsial’nykh uravnenii (Introduction to the Local Qualitative Theory of Differential Equations), St. Petersburg: St. Petersb. Gos. Univ., 2003 [in Russian].
3. Andronov, A.A., Leontovich, E.A., Gordon, I.I., and Maier, A.G., Qualitative Theory of Second-Order Dynamic Systems, New York: Wiley, 1973.