Filipkovska, M. S. “A Block Form of a Singular Pencil of Operators and a Method of Obtaining It”. Visnyk of V. N. Karazin Kharkiv National University. Ser. Mathematics, Applied Mathematics and Mechanics, Vol. 89, May 2019, pp. 33-58, doi:10.26565/2221-5646-2019-89-04.