Bogolyubov (juniour), N. N. ., & Prikarpatsky , A. K. . (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