Poslavskyi , S. and Rudnyev, Y. 2021. I. D. Borisov (obituary). Visnyk of V. N. Karazin Kharkiv National University. Ser. Mathematics, Applied Mathematics and Mechanics. 93, (Jun. 2021), 37-39. DOI:https://doi.org/10.26565/2221-5646-2021-93-05.