1. Dept. of Operation and Management Information Systems, Faculty of Business and Accountancy, Universiti Malaya, Kuala Lumpur 50603, Malaysia (Corresponding author), e-mail: thomas_yeong@yahoo.com
2. Inst. of Mathematical Sciences, Faculty of Science, Universiti Malaya, Kuala Lumpur 50603, Malaysia
3. School of Mathematical Sciences, Universiti Sains Malaysia, Penang 11800, Malaysia
4. Dept. of Physical and Mathematical Science, Faculty of Science, Universiti Tunku Abdul Rahman, Perak 31900, Malaysia