AlievB. A., KurbanovaN. K., and YakubovY. “On One Boundary-Value Problem for elliptic differential-Operator Equations of the Second Order With Quadratic Spectral Parameter”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 69, no. 6, June 2017, pp. 734-50, https://umj.imath.kiev.ua/index.php/umj/article/view/1732.