1. Department of Computer Science, University of Verona, Str. Le Grazie 15, Verona, IT-37131, Italy
2. Department of Mathematics and Institute of Applied Mathematics, Inha University, Incheon 402–751, Republic of Korea
3. Department of Mathematics, IGPM, RWTH Aachen University, Templergraben 55, Aachen, DE-52062, Germany