Nohara, B. T. “Nonexistence Theorem Except the Out-of-Phase and in-Phase Solutions in the Coupled Van Der Pol Equation System”. Ukrains’kyi Matematychnyi Zhurnal 61, no. 8 (August 25, 2009): 1106-1129. Accessed April 25, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3084.