BaranetskijYa. O., KalenyukP. I., and KolyasaL. I. 2018. “Spectral Properties of Nonself-Adjoint nonlocal boundary-Value Problems for the Operator of Differentiation of Even Order”. Ukrains’kyi Matematychnyi Zhurnal 70 (6), 739-51. https://umj.imath.kiev.ua/index.php/umj/article/view/1592.