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