1. Department of Mathematics, Deva Matha College, Kuravilangad-686 633, Kerala, India
2. Department of Mathematics, Pondicherry University, Puducherry-605 014, India
3. School of Applied Mathematics and Informatics, Hanoi University of Science and Technology, 1 Dai Co Viet, Hai Ba Trung, Hanoi, Vietnam
4. Department of Mathematical Sciences, Sharif University of Technology, P.O. Box 11155-9415, Tehran, Iran