A. Skučaitė, K. Skučaitė-Bingelė, S. Pečiulytė, and A. Štikonas, “Investigation of the spectrum for the Sturm–Liouville problem with one integral boundary condition”, NAMC, vol. 15, no. 4, pp. 501–512, Oct. 2010, doi: 10.15388/NA.15.4.14321.