1. Department of Mathematical Sciences and Research Institute of Mathematics, Seoul National University, Seoul 151-747, South Korea
2. Department of Mathematical Sciences, Seoul National University, Seoul 151-747, South Korea
3. Dipartimento di Meccanica, Matematica e Management, Politecnico di Bari, Via E. Orabona 4, 70125 Bari, Italy
4. Department of Civil Engineering, Design, Construction Industry and Environment, Second University of Naples, Via Roma 29, 81031 Aversa, Italy