1. Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 4, Sofia, 1113, Bulgaria
2. GRAPES, Beauvallon Res., r. Belle Jardiniere, 483 Liege Angleur, B-4031, Euroland, Belgium
3. Faculty of Economics, University of Tuscia, via del Paradiso 47, Viterbo, I-01100, Italy