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, Vol. 61, no. 8, Aug. 2009, pp. 1106-29, https://umj.imath.kiev.ua/index.php/umj/article/view/3084.