Koval’chuk, Y. A., and Y. V. Kozachenko. “Boundary-Value Problems With Random Initial Conditions and Functional Series from sub<sub class="a-Plus-plus">φ</Sub&gt; (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.