Palamut Kosar, N., and O. Akcay. “The Spectrum and Principal Functions of a Nonself-Adjoint Sturm–Liouville Operator With Discontinuity Conditions”. Eurasian Mathematical Journal, vol. 15, no. 3, Nov. 2024, pp. 55–67, https://emj.enu.kz/index.php/main/article/view/416.