LoveikinY. V., and ParasyukI. O. “Invariant Tori of Locally Hamiltonian Systems Close to Conditionally Integrable Systems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, no. 1, Jan. 2007, pp. 71–98, https://umj.imath.kiev.ua/index.php/umj/article/view/3292.