Bondarenko, G. I. “On Some Consequences of the Equation for the Markov Renewal Function of a Semi-Markov Process”. Ukrains’kyi Matematychnyi Zhurnal, vol. 56, no. 12, Dec. 2004, pp. 1684-90, https://umj.imath.kiev.ua/index.php/umj/article/view/3875.