1. Department of Mathematical and Actuarial Sciences, Universiti Tunku Abdul Rahman, Cheras, Kajang, Selangor, Malaysia; and Institute of Mathematical Sciences, University of Malaya, Kuala Lumpur, Malaysia
2. Department of Mathematics, Institute for Theoretical Computer Science, University of West Bohemia, Plzeň, Czech Republic
3. Institut für Computergraphik und Algorithmen, Technical University of Vienna, Austria