P. M. Kitanov and V. A. Plotnikov, “Bogolyubov theorem for quasidifferential equations with pulses”, Ukr. Mat. Zhurn., vol. 49, no. 11, pp. 1504–1511, Nov. 1997, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5149