LapshinA. L. “Equations for Second Moments of Solutions of a System of Linear Differential Equations With Random Semi-Markov Coefficients and Random Input”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 51, no. 6, June 1999, pp. 776–783, https://umj.imath.kiev.ua/index.php/umj/article/view/4664.