1. Department of Mathematical Sciences, Faculty of Science, Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia
2. Department of Mathematics, Faculty of Science, University of Kordofan, North Kordofan State, El-Obied 51111, Sudan
3. Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 31750 Tronoh, Malaysia