Koshlyakov, V. N. “On Structural Transformations of Equations of Perturbed Motion for a Certain Class of Dynamical Systems”. Ukrains’kyi Matematychnyi Zhurnal, vol. 49, no. 4, Apr. 1997, pp. 535–539, https://umj.imath.kiev.ua/index.php/umj/article/view/5028.