V. I. Urmanchev, “A stability criterion for periodic solutions of two-dimensional discontinuous dynamical systems”, Ukr. Mat. Zhurn., vol. 51, no. 9, pp. 1262–1266, Sep. 1999, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4721