Koval’chuk, Yu. A., and Yu. V. Kozachenko. 1998. “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 50 (4), 504–515. https://umj.imath.kiev.ua/index.php/umj/article/view/4920.