1. Department of Mathematics, Hanshan Normal University, Chaozhou, China
2. Department of Mathematics, Government Degree College Gulabad Dir (L), Khyber Pakhtunkhwa, Pakistan
3. School of Mathematical Science, Shanghai Jiao Tong University, Shanghai, P.R. China
4. Department of Mathematics, University of Malakand, Chakdara Dir (L), Khyber Pakhtunkhwa, Pakistan