Valeyev, K. G., and I. A. Dzhalladova. “Derivation of Moment Equations for Solutions of a System of Differential Equations Dependent on a Semi-Markov Process”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 54, no. 11, Nov. 2002, pp. 1569-73, https://umj.imath.kiev.ua/index.php/umj/article/view/4193.