1. Department of Mathematics, COMSATS University Islamabad, Attock Campus, Attock, Pakistan
2. Institute for Applied Mathematics, LS III TU Dortmund, Vogelpothsweg 87, 44227Dortmund, Germany
3. Department of Mathematics, COMSATS University Islamabad, Abbottabad Campus, Abbottabad, Pakistan
4. UGC Professor, University of Dhaka, Dhaka, Bangladesh