“On the existence and asymptotics of a periodic solution of a degenerate singularly perturbed system of differential equations in the case of multiple elementary divisors” (2002) Neliniini Kolyvannya, 5(1), pp. 123–141. Available at: https://twinhead.imath.kiev.ua/index.php/nosc/article/view/207 (Accessed: 22 January 2026).