Affiliation: | (1) Department of Mathematics & 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;(3) Faculty of Mathematics, University of Cluj, CP 253, 3400 Cluj, Romania; |