1. Department of Mathematics, Shanghai University, 99 Shangda Road, Shanghai 200444, P. R. China
2. Department of Electrical & Computer Engineering, Utah State University, Logan, USA
3. Industrial Engineering School, University of Extremadura, Avenida de Elvas s/n, Badajoz 06006, Spain
4. BERG Faculty, Technical University of Kosice, B. Nemcovej 3, 04200 Kosice, Slovakia