1. School of Electrical Engineering, Chungbuk National University, 1 Chungdae-ro, Seowon-gu, Cheongju 362-763, Republic of Korea
2. Department of Mathematics, Sri Ramakrishna Institute of Technology, Coimbatore, Tamilnadu 641010, India
3. Department of Mathematics, Sungkyunkwan University, Suwon 440-746, Republic of Korea