PortenkoN. I. “On Integro-Differential Equations for Distribution of Probabilities of Additive Functionals of Diffusion Processes ”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 18, no. 5, Aug. 1966, pp. 60-68, https://umj.imath.kiev.ua/index.php/umj/article/view/8441.