Anh, Nguyen Dong, and Nguyen Tien Khiem. “Sub Harmonic Oscillations in First Order Systems With Delay”. Vietnam Journal of Mechanics 12, no. 1 (March 30, 1990): 1–6. Accessed April 20, 2024. https://vjs.ac.vn/index.php/vjmech/article/view/10272.