Acikgoz M., Araci S., and ŞenE. “Spectral Problem for Sturm – Liouville Operator With Retarded Argument Which Contains a Spectral Parameter in Boundary Condition”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 68, no. 8, Aug. 2016, pp. 1102-14, https://umj.imath.kiev.ua/index.php/umj/article/view/1905.