1. Assistant Professor, Dept. of Mathematics, National Institute of Technology-Hamirpur, Anu Rd., Hamirpur, Himachal Pradesh 177005, India (corresponding author).
2. Professor, School of Mathematical Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia.
3. Professor, Faculty of Mathematics, Univ. of Cluj, R-3400 Cluj, CP 253, Romania.