Bogolyubov (juniour), N.N. and 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 (Jun. 1990), 794–800.