1. Faculty of Mathematics, University of Science and Technology Houari Boumediene, BP 32 El Alia, Bab Ezzouar, 16111 Algiers, Algeria
2. School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link 637371, Singapore
3. Department of Mathematics and Statistics, College of Arts and Sciences, American University of Sharjah, P. O. Box 26666 Sharjah, United Arab Emirates
4. Department of Mathematics and Statistics, Kenyon College, Gambier, Ohio OH 43022, USA