1. Ph.D. Candidate, Institute of Mathematical Sciences, Univ. of Malaya, 50603 Kuala Lumpur, Malaysia.
2. Researcher, Institute of Mathematical Sciences, Univ. of Malaya, 50603 Kuala Lumpur, Malaysia.
3. Professor, School of Mathematical Sciences, Faculty of Science and Technology, Univ. Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia.
4. Professor, Faculty of Mathematics, Univ. of Cluj, R-3400 Cluj, CP 253, Romania (corresponding author).