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: Ukr. Math. J., 54, No. 4, 693–699 (2002).
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: Ukr. Math. J., 51, No. 10, 1521–1524 (1999).
5. I. I. Klevchuk, “Existence of countably many cycles in hyperbolic systems of differential equations with transformed argument,” Nelin. Kolyv., 18, No. 1, 71–78 (2015); English translation
: J. Math. Sci., 215, No. 3, 341–349 (2016).