Chujko, E.V., Chujko, A.S. and Liubyma, O.E. 2012. An autonomous periodical problem for a Hill type equation. Visnyk of V. N. Karazin Kharkiv National University. Ser. Mathematics, Applied Mathematics and Mechanics. 1030 (Nov. 2012), 38-53. DOI:https://doi.org/10.26565/2221-5646-2012-1030-04.