1. Department of Mathematics, School of Physical Sciences, Federal University of Technology Owerri, P. M. B. 1526 Owerri, Imo State, Nigeria
2. Department of Mathematics, Government College University, 54000 Lahore, Pakistan
3. China Medical University, Taichung 404, Taiwan
4. Institute of Research and Development of Processes, University of the Basque Country, Campus of Leioa (Bizkaia), P.O. Box 644- Bilbao, Barrio Sarriena, 48940 Leioa, Spain