Department of Theoretical Physics, Physics Faculty, St. Kliment Ohridski University of Sofia, Sofia 1164, Bulgaria;
Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, Acad. G. Bonchev St. 8, Sofia 1113, Bulgaria
Prof. Stoytcho Yazadjiev
Department of Theoretical Physics, Physics Faculty, St. Kliment Ohridski University of Sofia, Sofia 1164, Bulgaria;
Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, Acad. G. Bonchev St. 8, Sofia 1113, Bulgaria