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4. M. A. Belozerova, "Asymptotic representations of solutions of nonautonomous second-order differential equations with nonlinearities close to power-type nonlinearities," Nelin. Kolyv., 12, No. 1, 3-15 (2009)
5. English translation: Nonlin. Oscillat., 12, No. 1, 1-14 (2009).