1. A.N. Sevchenko Institute of Applied Physical Problems, Belarusian State University, Kurchatov Str. 7, 220045 Minsk, Belarus
2. Belarusian State University, Nezavisimosti Ave. 4, 220030 Minsk, Belarus
3. Institute of Mathematics, National Academy of Sciences of Belarus, Surganov Str. 11, 220072 Minsk, Belarus
4. Friedrich-Schiller University Jena, Max-Wien-Platz 1, 07743 Jena, Germany