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3. Yu. A. Mitropol’skii, A. M. Samoilenko, and D. I. Martynyuk, Systems of Evolution Equations with Periodic and Almost Periodic Coefficients [in Russian], Naukova Dumka, Kiev (1985).
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5. G. P. Pelyukh, “On the behavior of solutions of a system of nonlinear difference equations,” in: Dynamical Systems and Problems of Stability of Solutions of Difference Equations [in Russian], Institute of Mathematics, Ukrainian Academy of Sciences, Kiev (1973), pp. 73–80.