PrykarpatskyA. K., SamoilenkoA. M., and TimchishinO. Y. “The Poincare-Mel’nikov Geometric Analysis of the Transversal Splitting of Manifolds of Slowly Perturbed Nonlinear Dynamical Systems. I”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 45, no. 12, Dec. 1993, pp. 1668–1681, https://umj.imath.kiev.ua/index.php/umj/article/view/5975.