1. School of Mathematical Sciences, Universiti Sains Malaysia, Penang, Malaysia;
2. Department of Physical and Mathematical Science, Faculty of Science, Universiti Tunku Abdul Rahman, Kampar, Perak, Malaysia;
3. School of Mathematical and Computer Sciences, Heriot-Watt University Malaysia, Putrajaya, Malaysia;
4. Université de Nantes & LS2N UMR 6004, Nantes, France