1. Department of Mathematical Modelling, Technical University of Denmark, DK–2800 Lyngby, Denmark
2. Department of Mathematics, Heriot-Watt University, Edinburgh EH14 4AS, UK
3. Theoretical Physics Division, NASU Institute of Magnetism, 36-b Vernadsky str., Kiev-680, 252142, Ukraine
4. Institute of Electronics, Bulgarian Academy of Sciences, Boulevard Tsarigradsko shosse 72, Sofia 1784, Bulgaria