AlievB. A., KurbanovaN. K., and YakubovYa. 2017. “On One Boundary-Value Problem for elliptic differential-Operator Equations of the Second Order With Quadratic Spectral Parameter”. Ukrains’kyi Matematychnyi Zhurnal 69 (6), 734-50. https://umj.imath.kiev.ua/index.php/umj/article/view/1732.