Nguyen Tien Khiem. “A New Approach to Solution of the Focker-Plank-Kolmogorov Equation Tor Random Oscillatory Nonlinear Systems”. Ukrains’kyi Matematychnyi Zhurnal, vol. 44, no. 8, Sept. 1992, pp. 1123-9, https://umj.imath.kiev.ua/index.php/umj/article/view/8045.