a Moscow Institute of Physics and Technology (State University), Dolgoprudnyi, Moscow oblast, 141700 Russia
b Mechanics and Mathematics Faculty, Moscow State University, Moscow, 119991 Russia
c Institute of Mathematics and Computer Science, Buryat State University, Ulan-Ude, 670000 Buryat Republic, Russia
Correspondence to: *e-mail: mraigor@yandex.ru
Correspondence to: **e-mail: xp1@protonmail.com
Received 25 August, 2017
DOI: 10.1134/S1064562418010155