Sobolev Institute of Mathematics, Siberian Branch, Russian Academy of Sciences, pr. Akademika Koptyuga 4, Novosibirsk, 630090 Russia
Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090 Russia
Correspondence to: e-mail: kopylov@math.nsc.ru
Received 19 September, 2016
DOI: 10.1134/S1064562416060120