Chujko, E. V., A. S. Chujko, and O. E. Liubyma. “An Autonomous Periodical Problem for a Hill Type Equation”. Visnyk of V. N. Karazin Kharkiv National University. Ser. Mathematics, Applied Mathematics and Mechanics, no. 1030, Nov. 2012, pp. 38-53, doi:10.26565/2221-5646-2012-1030-04.