Anh, Nguyen Dong, and Nguyen Tien Khiem. “Sub Harmonic Oscillations in First Order Systems With Delay”. <i>Vietnam Journal of Mechanics</i> 12, no. 1 (March 30, 1990): 1–6. Accessed January 11, 2026. https://vjmech.vast.vn/vjmech/article/view/10272.