1. Department of Mathematics and Institute
for Mathematical Research, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
2. School of Mathematical Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia e-mail:
3. Department of Mathematics, Babeş-Bolyai University, 400084 Cluj-Napoca, Romania