1. Department of Mathematical Physics, Faculty of Applied Mathematics, Belarussian State University, Nezavisimosti Ave. 4, 220030 Minsk, Belarus
2. Department of Mathematical Sciences, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom
3. Department of Mathematics and Statistics, UiT the Arctic University of Norway, Tromsø 90-37, Norway
4. Department of Mathematics and Natural Sciences, University of Stavanger, 40-36 Stavanger, Norway