UrmanchevV. I. “A Stability Criterion for Periodic Solutions of Two-Dimensional Discontinuous Dynamical Systems”. Ukrains’kyi Matematychnyi Zhurnal 51, no. 9 (September 25, 1999): 1262–1266. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4721.