The research of SA has been partially supported by GNFM-INDAM
and RFO University of Bologna. The work of PGG was partially supported by the Russian Foundation
of Basic Research grant No 20-01-00157. We acknowledge the financial support of the project MMNLP
(Mathematical Methods in Non Linear Physics) of the INFN.
Поступила в редакцию: 07.09.2021 Исправленный вариант: 30.07.2022 Принята в печать: 03.11.2022