Aliev, A. R. “On the Solvability of a Fourth-Order Operator-Differential Equation With Multiple Characteristic”. Ukrains’kyi Matematychnyi Zhurnal 66, no. 5 (May 25, 2014): 699–707. Accessed April 26, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2171.