1. Mathematisch InstituutUniversiteit Leiden, Niels Bohrweg 12333 CA, Leiden, Netherlands
2. Universitas Gadjah MadaBulaksumur, Daerah Istimewa Yogyakarta, Indonesia
3. Department of Management Science and Information SystemsRutgers Business SchoolNewark and New Brunswick, 1 Washington Park, Newark, NJ 07102, USA