I. N. Shchitov, “Asymptotic expansions of solutions to singularly perturbed systems”, Ukr. Mat. Zhurn., vol. 45, no. 4, pp. 552–560, Apr. 1993, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5842