1. Department of Mathematics Education, National Taichung University of Education, Taichung 403, Taiwan
2. Instituto Universitario de Matemática Pura y Aplicada, Universitat Politècnica de VaÌencia, E-46022, València, Spain
3. Faculty of Technical Sciences, University of Novi Sad, Trg D. Obradovića 6, 21125 Novi Sad, Serbia
4. Departamento de Matemáticas, Universitat Jaume I, Campus de Ríu Sec, E-12071, Castelló de la Plana, Spain