Bogolyubov (juniour), N. N., and A. K. Prikarpatsky. 1990. “Bilocal Periodic Problem for the Sturm-Liouville and Dirak Operators As Well As Certain Applications in the Theory of Nonlinear Dynamic Systems. I ”. Ukrains’kyi Matematychnyi Zhurnal 42 (6): 794-800. https://umj.imath.kiev.ua/index.php/umj/article/view/8839.