AleksandrovichI. N., and SidorovM. V. “Differential Operators Specifying the Solution of an Elliptictype Iterated Equation”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 71, no. 3, Mar. 2019, pp. 433-40, https://umj.imath.kiev.ua/index.php/umj/article/view/1449.