1. Department of Mathematics, Shah Jalal University of Science and Technology, Sylhet–3114, Bangladesh
2. Department of Physiology, 3655 Promenade Sir William Osler, McGill University, Montreal, Quebec H3G 1Y6, Canada
3. Department of Mathematics and Statistics, University of Victoria, P.O. Box 1700 STN CSC, Victoria, British Columbia V8W 2Y2, Canada