Prykarpatsky, Y. A., & Samoilenko, A. M. (1999). Investigation of invariant deformations of integral manifolds of adiabatically perturbed completely integrable hamiltonian systems. I. Ukrains’kyi Matematychnyi Zhurnal, 51(10), 1379–1390. https://umj.imath.kiev.ua/index.php/umj/article/view/4737