ANH, N. D.; KHIEM, N. T. Non-linear oscillations in first order systems with delay. II. Non-autonomous case. Vietnam Journal of Mechanics, Hanoi, Vietnam, v. 12, n. 3, p. 1–7, 1990. DOI: 10.15625/0866-7136/10282. Disponível em: https://vjs.ac.vn/index.php/vjmech/article/view/10282. Acesso em: 24 apr. 2024.