1. a Department of Mathematics, Faculty of Science , Mahidol University , Rama 6 Road, Bangkok, 10400, Thailand
2. b Centre of Excellence in Mathematics, PERDO , Bangkok, 10400, Thailand
3. c Departamento de Ciencias Basicas , Tecnologico de Monterrey , Campus Guadalajara, Zapopan, Jalisco C.P., 45201, Mexico
4. d Department of Mathematics , Washington State University , Pullman, WA, 99164-3133, USA