1. School of Mathematical Sciences, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
2. Department of Mathematics and Statistics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia
3. Institute of Engineering Mathematics, Universiti Malaysia Perlis, Kampus Pauh Putra, 02600 Arau, Perlis, Malaysia
4. School of Distance Education, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia