1Department of Mathematical and Actuarial Sciences, Universiti Tunku Abdul Rahman, Jalan Genting Kelang, 53300 Setapak, Kuala Lumpur, Malaysia
2Institute of Mathematical Sciences, University of Malaya, 50603 Kuala Lumpur, Malaysia
3NUS High School of Maths. & Science, 20 Clementi Avenue 1, Singapore, 129957