1. Mathematics Section, School of Distance Education, Universiti Sains Malaysia, 11800 USM, Pulau Pinang, Malaysia
2. School of Mathematical Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia
3. Department of Mathematics, Babes¸-Bolyai University, R-400048 Cluj-Napoca, Romania