KubichkaA. A., and ParasyukI. O. “Bifurcation of a Whitney-Smooth Family of Coisotropic Invariant Tori of a Hamiltonian System under Small Deformations of a Symplectic Structure”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 53, no. 5, May 2001, pp. 610-24, https://umj.imath.kiev.ua/index.php/umj/article/view/4284.