\Bibitem{1} \paper Nina Nikolaevna Subbotina \serial Trudy Inst. Mat. i Mekh. UrO RAN \yr 2017 \vol 23 \issue 1 \pages 5--7 \mathnet{http://mi.mathnet.ru/timm1378} \crossref{https://doi.org/10.21538/0134-4889-2017-23-1-5-7} \elib{https://elibrary.ru/item.asp?id=28409362}