Unpublished work

This work was presented at Continuum Mechanics Seminar (Семинар по Механике Сплошных Сред) at the Institute of Mechanics (Moscow State University) on March 4, 2020; and submitted for publication to Computational Mathematics and Mathematical Physics  (Журнал вычислительной математики и математической физики) in August 2020. While the article was under review,  the author passed away in February 2021.  See English translation.