1. Ph.D. Scholar, Dept. of Mathematics, Quaid-I-Azam Univ. 45320, Islamabad 44000, Pakistan (corresponding author).
2. Professor, Dept. of Mathematics, Faculty of Science, King Abdulaziz Univ., P.O. Box 80257, Jeddah 21589, Saudi Arabia.
3. Professor, Dept. of Mathematics, Quaid-I-Azam Univ. 45320, Islamabad 44000, Pakistan; and Dept. of Mathematics, Faculty of Science, King Abdulaziz Univ., P.O. Box 80257, Jeddah 21589, Saudi Arabia.