1. S. Pianskool, A. Wasanawichit, Divisibility of Certain Periodic Semigroups, Southeast Asian Bulletin of Mathematics., 29 (2005), 807-812.
2. S. Pianskool, A. Wasanawichit and Y. Kemprasit, Certain divisible hypergroups, General Mathematics (Romania), 13(3)(2005), 15-22.