1. Department of Mathematics, Faculty of Science, Northern Border University, P.O. Box 1231, Arar 91431, Saudi Arabia
2. Department of Applied Mathematics, Faculty of Mathematical Sciences, University of Khartoum, P.O. Box 321, 11111 Khartoum, Sudan
3. Department of Physics, Faculty of Science, University of Khartoum, P.O. Box 321, 11111 Khartoum, Sudan