Apakov, Y. P., and T. D. Dzhuraev. “On the Theory of the Third-Order Equation With Multiple Characteristics Containing the Second Time Derivative”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 62, no. 1, Jan. 2010, pp. 40 -51, https://umj.imath.kiev.ua/index.php/umj/article/view/2842.