1. Institut für Angewandte Mathematik, Universität Bonn, Wegelerstr. 6, D-53115 Bonn, Germany
2. Institute of Mathematics and Information Technologies, Uzbekistan Academy of Sciences, Dormon Yoli str. 29, 100125, Tashkent, Uzbekistan
3. The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy
4. Karakalpak State University, Ch. Abdirov str. 1, 142012, Nukus, Uzbekistan