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, [S. l.], v. 61, n. 8, p. 1106–1129, 2009. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3084. Acesso em: 6 dec. 2025.