Koval’chuk, Yu. A., and Yu. V. Kozachenko. “Boundary-Value Problems With Random Initial Conditions and Functional Series from Subφ (Ω). I”. Ukrains’kyi Matematychnyi Zhurnal, vol. 50, no. 4, Apr. 1998, pp. 504–515, https://umj.imath.kiev.ua/index.php/umj/article/view/4920.