1. Institute of Engineering, Polytechnic of Porto Dept. of Electrical Engineering Rua Dr. António Bernardino de Almeida, 431 4249 – 015 Porto, Portugal
2. Department of Physics University of Bologna, and INFN-Bologna Via Irnerio 46, I-40126 Bologna, Italy
3. Institute of Mathematics and Informatics Bulgarian Academy of Sciences, “Acad. G. Bontchev” Str., Block 8 Sofia–1113, Bulgaria
4. Department of Mechanics, Faculty of Technical Sciences University of Novi Sad 21000 Novi Sad, Serbia