KOVAL’CHUK, Y. A.; KOZACHENKO, Y. V. 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, v. 50, n. 4, p. 504–515, 25 Apr. 1998.